We studied the breeding conditions in medaka (Oryzias latipes), in order to establish a testing protocol for evaluating the reproductive toxicity of PPCPs (Phamaceuticals and Personal Care Products). Medaka at about 60-day posthatch were cultured at groups with different sex ratios for 28 days, and the days of first spawning, duration of spawning, and fecundity of each group were investigated. It was found that medaka has the ability of reproduction at about 60-day posthatch under optimal environmental conditions, and the number of eggs per female increased with aging and reached its peak at around 160-day posthatch. When a few males lived with females in one aquarium, they often did not initiate mating behavior. Also, one male tended to mate up to three females at a time. Furthermore, in order to examine the effect of water temperature elevation on breeding in medaka, mating pairs (8 pairs/group) were cultured at 24℃ or 28℃ for 21days, and the spawning activity was obseved. All pairs which were cultured at 28℃ spawned almost every day. On the other hand, one out of 8 pairs at 24℃ did not spawn throughout the observation period, and the day of first spawning in another pair was extremely delayed, resulting in significant variability in the number of total eggs spawned by each pair. These results suggest that the pair-culture at 28℃ is more appropriate for breeding of medaka.