The reproductive processes of the Japanese Medaka, Oryzias latipes, have been studied with a view to comparing them with other poikilothermous forms and also to attempt to control the ovulation process for practical purposes. The [female] urinogenital system is much like that of other poeciliid fish. There is an impaired and hollow ovary having many developing follicles on all but the dorsal side. The mature eggs rupture into the lumen of the ovary and pass quickly through a short, muscular, tubular oviduct to the genital papilla. The ovulation process takes less than 30 minutes but is not cataclysmic, and a single [female] may ovulate up to 80 eggs a single morning. Copulation occurs during oviposition. The fish normally ovulate before dawn, and inverting the periods of light and darkness causes the ovulation time to be shifted to the time of artificial dawn, indicating some relation of the light cycle to ovulation time. It is suggested that light governs the time of ovulation by regulating the general metabolic activity of the [female], so that the eggs are ovulated at the end of a period of relative quiescence.