100.3 was the FM sister station of Alton, IL's WOKZ (1570 kHz), and Broadcasting Yearbook gives the starting date in 1961. The AM was a daytimer and did a full simulcast with the FM, with the FM signing off at midnight. WOKZ AM and FM was an MOR (adult standards mixed with some AC).
The owner of R&B formatted KATZ purchased WOKZ-FM and began operating the station on New Year's Eve 1978. The call letters became WZEN. The new station opened by playing disco into the New Year of 1979. For the first four months, the station had an Adult Contemporary format. In April, the station went to an all Disco format and was known as "Disco 100" through the rest of 1979. With Disco falling out of favor by the end of the year, the station shifted into dance music, some new wave and Top-40. By February 1981, WZEN dropped this format and became an R&B station similar to KATZ. By the end of 1981, WZEN tweaked its format, and approached R&B with an AOR/freeform approach, and became known as 'Z-100.' Different jocks tended to have a slightly different sound to their shows, compared to more mainstream Urban stations like KMJM/Majic 108, or R&B stations like WESL and KATZ. Z-100 also introduced the "Roll Call" show, that featured callers to rap on the air.
In 1986, WZEN became KATZ-FM, and the programming began to reflect more of a standard R&B format, moving in an Urban AC direction
In 1990, The station flipped formats to a mix of smooth jazz and R&B known as "Jazz 100." This format would last three years before returning to an Urban AC format. Smooth jazz returned in 1994 as KNJZ and lasted until September 1995, and afterwards returned to Urban AC. The station flipped to R&B/Hip Hop in 1999 as "The Beat."