Although numerous studies have documented changes in the behavior of nonhuman primate females around the time of ovulation, very little attention has been paid to behavior changes around the time of menstruation. Yet such information is obviously relevant to under standing the origins and etiology of the adverse mood and behavior changes experienced premenstrually by many women. Analysis of data obtained during 115 hours of observation on 13 female Amboseli (Kenya) baboons (Papio cynocephalus) during 24 menstrual periods showed that prior to and during the time of menstrual onset, these individuals exhibited distinct changes in their activity budgets, nearest-neighbor distances, and patterns of social interaction. Furthermore, in comparison to females around the time of ovulation, perimenstrual females showed increased rates of agonistic interaction and decreased (but nonzero) rates of sexual interaction with adult males. These premenstrual and perimenstrual behavior changes among female yellow baboons thus show some intriguing similarities to several commonly reported behavioral symptoms of premenstrual syndrome (PMS) in women.