1. Juvenile female ayu (N = 400), Plecoglossus altivelis, were assigned to each of four different photoperiod/temperature regimes (16 L/18-degrees-C, 16 L/24-degrees-C, 8 L/18-degrees-C and 8 L/24-degrees-C) for 16 weeks. 2. Mean body weight in the 16 L/18-degrees-C treatment group was the highest of the four treatment groups. No significant differences between body weights of the 16 L/24-degrees-C, 8 L/18-degrees-C and 8 L/24-degrees-C treatment groups were observed. 3. Ovulation and the highest values of gonadosomatic and hepatosomatic indices, plasma testosterone (T) and estradiol-17beta (E2) occurred in the 8 L/18-degrees-C group. The levels of plasma T or E2 were inconsistent in order among the 16 L/18-degrees-C, 8 L/24-degrees-C and 16 L/24-degrees-C treatment groups. 4. Peak levels of 17alpha-hydroxyprogesterone occurred in the 8 L/18-degrees-C and 8 L/24-degrees-C groups. 5. The data suggest that gonadal development in ayu is photoperiod and temperature dependent: short photoperiod and cool temperature favour gonadal development.
