There was no question yesterday what the No. 1 hit on a Fairfax County radio station was, but there is still the question of who was broadcasting it.

At 9:14 yesterday morning, amidst the normally crisp, cryptic chatter on the county fire department's emergency radio channel No. 1, the strains of pop singer Madonna burst forth:

"Like a virgin

"Touched for the very first . . ."

And then Madonna was interrupted as a dispatcher sent rescue equipment to aid someone suffering chest pains. But as soon as the dispatcher stopped talking, Madonna hit the airwaves again.

After a full minute of music, the dispatcher ordered "All units on Channel 1, switch to Channel 2." As fire and rescue units went to the alternate channel, officials scrambled to discover the source of Madonna.

Fire Capt. James Dundas said the department was baffled. He heard Madonna -- whom he doesn't know from Prince -- in his office and ordered the switch in frequencies.

Although accidental interference on the emergency radio channels happens occasionally, Dundas said the quality and strength of yesterday's broadcast led him to believe it was a deliberate prank.

Interference usually comes when a microphone becomes stuck in the transmit position, or when someone is deliberately playing in an unattended emergency vehicle.

Bob Merlis, a vice president at Warner Bros. records, said the unauthorized broadcast didn't surprise him. "It's been said that Madonna's career is on fire, after all," he deadpanned