The South American squirrel monkey (Saimiri) shows pronounced annual variations in reproductive activity. We investigated the contribution of social stimulation to breeding readiness, as reflected in behavior and gonadal hormones. A previous study indicated that formation of new heterosexual pairs can induce breeding readiness in males, but not in females. In the present study we examined the response of individually housed females to the formation of triads of unacquainted females and, subsequently, the response of these triads to the introduction of a single male. The formation of the all-female group was accompanied by a marked reduction in basal cortisol levels. Introduction of a male to such a group promptly induced breeding readiness in the females, as indicated by increased gonadal hormone production and cyclic variation in estrogen and progesterone levels. The contrasting responses of group-living or single females to a new male suggest that at least two processes are involved in the induction of breeding readiness by group formation in females. One is a "priming" effect resulting from interfemale stimulation: the other is a "triggering" effect resulting from the presence of a male.