SummaryThe seasonal or cyclical changes in the size, histology and secretions of the male reproductive organs and accessory glands of certain mammals are reviewed briefly. Some mammals such as man and the laboratory rat are fertile throughout the year whereas others such as the Red deer stag and the mole are fertile only at certain periods. The male's cycle cannot necessarily be inferred from the female's cycle in those mammals that have a seasonal cycle, neither can it be inferred from rutting or copulatory behaviour. In some species the male is fecund throughout the year whereas the female has a well defined breeding season or seasons, which is defined as the period between conception and parturition. It is stressed that morphological changes, whether macroscopic or histological, do not necessarily reflect the secretory activity of the gland and that the presence of spermatozoa indicates fecundity, not fertility.The information available on the reproductive organs, accessory glands and breeding cycles of mammals which occur in Britain is summarized. The majority of these mammals exhibit a seasonal cycle. The reproductive cycle of Fallow, Red and Sika deer, which are temperate species, can be correlated with the shedding and growth of antlers; for Red and Sika deer this is controlled by changes in the photoperiod. Two tropical mammals, the wildebeest and the hippopotamus, are considered and in the wildebeest we may have an animal which has a well–defined rutting period although spermatogenesis proceeds throughout the year. Although the male hippopotamus is sexually active throughout the year, there are peaks in the calving season which can be correlated with the rainy seasons. In some hibernating insectivores and rodents, recrudescence of the gonads and accessory glands begins before hibernation finishes. In temperate bats insemination usually takes place in the Autumn and this is followed by hibernation, fertilization not taking place until the Spring. Fertilization in some species ofMiniopterustakes place after copulation but growth of the embryo is retarded during hibernation.Finally, the causes of the cyclical waxing and waning of the reproductive organs and accessory glands and the effects of various external stimuli are considered. Very little experimental work has been done in this field apart from that on the effect of light on some domesticated specie