The early and late effects of a single whole-body exposure of 400 r X-ray on reproduction were studied in rats exposed during various stages of the oestrous cycle prior to mating. The early effects, i.e. those observed in females that mated during the first expected oestrus after irradiation, seemed to involve superovulation, the greatest response occurring in females irradiated during metoestrus (M) or early dioestrus (M+l). Implantation rates were increased in all groups of females except those which mated half a day after irradiation (M+3B). Average litter size was highest in females irradiated during metoestrus or early dioestrus. Those females irradiated during late dioestrus (M+3 and M+3A) had litters which were comparable to control size, whereas the litters were more or less completely re-sorbed in females that mated half a day after irradiation. Though the numbers of corpora lutea were still increased up to at least 171/2 days after exposure, the number of implantations, and of 20-day foetuses, was comparable to control values In females that mated after the first expected oestrus (late effects). Foetuses, under both early and late effects, were normal in size and development. The physiological implications of this phenomena are discussed.