Adult female medaka, Oryzias latipes, with regressed ovaries were exposed to 7 h of continuous light plus an additional hour of light at different times during the dark period of a 24-h cycle (7 + 1 light: 16 dark) at 24 ± 1 °C. When this additional hour of light falls on the 16th h, counting the onset of the light period as the 0 h, ovarian development was induced. This ovarian development is enhanced when the exposure period is increased and when the fish are preexposed to a warm temperature. The use of a photosensitive daily rhythm to measure photoperiod and to time the spawning season in the medaka is suggested.