SummaryStudy of the causation of seasonal breeding can be subdivided into a consideration of external or ecological, and internal or physiological, factors. Of the ecological ones, the role of day–length has been most extensively studied. Subjecting appropriate animals to different photoperiods should give fresh insight into mechanisms of the central nervous system. Alterations in day–length may produce changes in the C.N.S. and pituitary quite quickly, say within 24 to 48 hours. But there is still uncertainty about such basic problems as the relative significance of abrupt and gradual changes in day–length for the alteration of gonadal activity; and the existence of an annual endogenous physiological rhythm which may provoke the gonads into activity at the appropriate time. In addition, for some small rodents physical environmental factors apparently are not always equally efficient in stopping breeding in the winter, and it may be that length of breeding season is also influenced by biotic factors, such as the phase of a population cycle. Susceptibility to the effects of short photoperiods (“winter”) may be to some extent genetically controlled. Possibly then genotypes allowing winter breeding may be favoured at some, but not other, phases of populati