NYONE who has ever made a mix tape to play on a long drive knows the hypnotic effect of combining cars, motion and music: it's just you and the road and Steve Earle cranked up all the way as the mile markers fly by and stress melts away. Even when stuck at a traffic light, you can car dance by wiggling around in your seat and singing the background vocals on "Stop! In the Name of Love."

People have been piping tunes into the dashboard for decades. Radio became popular in cars in the 1930's; later came a rush of tape decks and CD players. It occurred to me not long after I got immersed in the world of MP3 music that this was a perfect format for the car.

Even the smallest flash-memory MP3 players can hold an hour or two of music, and hard-drive-based players like the Archos Jukebox or the Apple iPod can hold thousands of songs. A couple of roadworthy playlists could save me the frustration (and bad habit) of fiddling with the radio or fishing around for the right Talking Heads album while driving. I thought it would be great to hook up my portable MP3 jukebox player to a car stereo on my next long trip.

Looking for options, I found plenty. There are all kinds of devices, including cassette adapters that connect the MP3 player to (and allow it to play through) the car's tape deck, and new CD players that can spin a disc with several hours of MP3's as you drive along. The low and the high end of the spectrum caught my eye, however.

On the low end, I discovered an array of inexpensive wireless FM transmitters being marketed to the iPod/Rio/Nomad crowd as a way to play MP3 files through a car radio without having to bother with clumsy wiring or ill-fitting cassette adapters. On the high end, I discovered the PhatNoise Digital Car Audio System, a full-blown MP3 jukebox (the PhatBox) that uses a 20-gigabyte hard drive stored in the trunk to let you tow 500 CD's worth of tunes wherever you go.

I started with the FM transmitters: a SoundFeeder SF121 from Arkon Resources ($25; www.arkon.com), the iRock 300W by First International Digital ($30; www

.myirock.com), and the iTrip by Griffin Technology ($35; www.griffintechnology

.com).

All three of them worked basically the same way: you connect the FM transmitter's plug to the portable MP3 player's headphones port, find an unoccupied radio frequency on the FM dial for the transmitter to borrow, and push the Play button to broadcast your MP3 playlists through the car radio, the home stereo or even a portable radio.

I have MP3's scattered over several devices, including my Sony Clié organizer and my iPod, which I used for testing. The SoundFeeder SF121 and the iRock 300W run on a pair of AAA batteries and have short cables that can plug into music players that use the standard 3.5-millimeter stereo mini-plug for headphones; the iTrip is a glossy tube of white plastic that was designed to look good and work with the Apple iPod.

I tried the iRock 300W first, over Memorial Day weekend in Pennsylvania. You can tune its transmitter to one of four FM frequencies, 88.1 through 88.7, but if you live in an area with radio stations on these channels, the small choice of frequencies could pose a problem. Finding usable frequencies around New York City later was a challenge, but in the Pocono Mountains, I soon had the title track from "A Mighty Wind" on my Sony Clié pouring through the dashboard radio in a friend's S.U.V.

The SoundFeeder SF121 works similarly, although you have a greater choice of radio frequencies, from 88 to 95 megahertz. Its simple black-box design is not as energetic as the iRock 300w's contoured mix of opaque white plastic and gray rubber, but looks do not matter that much if you are going to have the transmitter and MP3 player tucked in the car's cup holder.

The iTrip is a little more complicated to set up but gives you a much wider range of radio frequencies to use. After you install the iTrip software on your computer and synchronize the PC with the iPod, the player gets a new playlist called iTrip stations with tiny audio files named after frequencies, from 88.1 to 107.9.

The sound through all three of the FM transmitters could be passable - anathema for hard-core audiophiles who can even hear the timpani player scratch his nose between measures on a recording of the Hallelujah Chorus, but O.K. for the listener who just wants a diversion from the traffic at the Holland Tunnel. The tone of the music was flattened, but if you can hold a frequency, you can avoid commercial radio programming.

All of the FM transmitters worked as advertised, and they have the advantage (for a car renter like me) of being portable. But they are a cheap and imperfect solution, mostly because of the nature of radio itself. The reception on each device could start strong, but if you drift into a new area, your ability to use an empty frequency clearly suffers, and so does the intelligibility of your music when the Static Monster butts in.

I jumped next to the other end of the MP3-in-the-car spectrum. The PhatNoise jukebox has been around for a few years, but the company recently announced deals with Volkswagen and Audi to include the system as a $795 option in new cars. The PhatNoise system can be added to many Volkswagen and Audi models and will work in other cars with certain stereos; the compatibility page at www.phatnoise

.com has specifics and prices.

In the Volkswagen Passat W8 that I borrowed for the evaluation, the PhatBox itself - a Linux-based digital media player hardware and removable 20-gigabyte cartridge - fit snugly and discreetly into its own compartment in the car's trunk. The player was controlled from the dashboard's existing CD changer: to search and play tracks stored on the hard drive, I pressed the buttons on the CD changer to search for music by track, playlist, album, artist and genre. To load new music, you just pop the hard drive out of the trunk, snap it into a U.S.B. 2.0 cradle attached to your computer and copy the tunes you want from your MP3 collection.

The PhatNoise system provides its own software for ripping and transferring MP3 tracks; you can also use tracks from your library.

The system uses audio navigation cues, so while the CD display screen will not show the typical MP3 song information, a synthesized male voice will speak up and announce the names of the artists, albums, genres and playlists. This system is good for keeping your eyes on the road, although it feels a bit as if Mr. Moviefone's serious younger brother is riding shotgun. Duplicate volume and album-navigation controls are built into the Passat's steering wheel, a wonderful touch.

I drove around New York with two friends to give the PhatNoise system a listen. We played a variety of music, including Billie Holiday, Radiohead and the Blue Man Group, with some salsa mixed in. Although the MP3 music lacked the pristine clarity of a compact disc, the tracks (encoded at 192 kilobits per second) blasting out of the Passat's Monsoon speaker system sounded good. Having so much music available at the push of a button reinforced my feeling that MP3 is a great format for car audio.

As a Manhattan pedestrian, though, I will have to settle for walking really fast up Broadway with my iPod headphones on while trying to get my music in motion. Somehow, that's just not the same as barreling down the highway with Janis Joplin up and the windows down.