The social interactions of 12 adult Lemur catta were observed under standard conditions over a 20-month period. Descriptions of the behavioural repertoire and of vaginal cytology throughout two breeding seasons provided the basis for a preliminary analysis of hormonal influences on social behaviour in a“lower”primate. Both long and short term fluctuations in gonadal activity were associated with changes in the frequency of expression of several non-sexual patterns, particularly of the species-characteristic and sexually differentiated displays of males. The latter appear to be predominantly olfactory gestures and serve to distribute skin-gland secretions, thereby integrating much social activity. In contrast to the extended receptivity characteristic of some monkeys and apes, mating in the Ring-tail is restricted to a brief period during the 51/2-week cycle, being invariably coincident with vaginal oestrus. Hormonal factors alone are probably sufficient to determine the appearance of full behavioural oestrus. Patterns (other than fighting) tentatively associated with territoriality and/or dominance appear the least sensitive to influence by gonadal secretions.