The Federal Communication Commission is demanding from the nation's big four radio companies answers to questions about their music playlist policies.

The US media watchdog has sent letters to CBS Radio, Clear Channel Communications, Citadel Broadcasting and Entercom Communications asking for information about gifts and payments received from record labels, and how songs get to be added to stations' playlists.

The four have declined to comment and all have previously stated that they adhere to the rules, which require that radio stations must disclose on air if they are paid to play a song.

The payola nut is a difficult one to crack, despite loud protestations by the major radio stations that they have cleaned up their act.

Independent music promoters - once allegedly the worst offenders - may now be shunned, but record labels consistently sponsor shows, offer competition prizes and perks for station staff. This, claim critics, adds up to the same as pay-for-play.

The FCC's turning up the heat on radio companies may not be entirely unconnected with New York attorney general Eliot Spitzer's recent complaints it is not doing enough to enforce its regulations.

Data sourced from Wall Street Journal Online; additional content by WARC staff