Seasonal changes in gonadal size, body weight, male song and the plasma titres of luteinizing hormone (LH), oestradiol and testosterone were investigated in a free-living population of African stonechats, Suxicola torquata axillaris, on Menengai Crater, Nakuru, Kenya. These were compared with the occurrence of nesting and moult in the population and a number of seasonally changing environmental factors including photoperiod, insect abundance and rain. Annual cycles in these stonechats appears to be regulated by endogenous events which are responsive to environmental factors on a seasonal basis. The dry season may play an important rôle in the synchronization of the population by inhibiting nesting in birds otherwise capable of reproductive activity. Rain is postulated as a time-giving cue for moult and gonadal regression. The possibility of photoperiodic changes at the equator acting as seasonal time-giving cues remains open.