In 1943, the station's call sign was changed to WWZR. In 1946, the station's call sign was changed to WEFM, and it began broadcasting on 98.5 MHz, while continuing to broadcast at 45.1 MHz. WEFM's call letters stood for the initials of Zenith Radio Corporation president Eugene F. McDonald. In 1947, the station's frequency was changed to 99.5 MHz. On June 1, 1961, WEFM became the second station in the United States to broadcast in FM stereo. In 1972, its transmitter was moved to the John Hancock Center.