Regressed female medakas were injected with synthetic luteinizing hormone–releasing hormone (LH–RH) intraperitoneally twice a week for 6 weeks under warm temperature (23 ± 1 °C) and short photoperiod (8 h light: 16 h dark). A dose response was observed as shown by the gonadosomatic index and the percentage distribution of class III oocytes with yolk formation. Ovulation also occurred in fish injected with the highest dose. Pituitary cytology revealed stimulation of gonadotropic cells in the proximal pars distalis by synthetic LH–RH at doses of 100 and 1000 ng per gram body weight. Thus, synthetic LH–RH seems to be effective in inducing ovarian development in the medaka. In fish, the presence of a system similar to the mammalian LH–RH system seems likely.