Behavioral changes associated to the menstrual cycle in a social group of vervet monkeys (Cercopithecus aethiops) were studied. Three adult females were used as experimental subjects and in these, vaginal smears were taken every other day in order to detect their menstrual cycles. Only the dominant and the mid-ranking female showed regular cycles while the low-ranking female showed amenorrhea. The menstrual cycles were divided into five periods (menstrual, premenstrual, luteal, ovulatory and follicular) which were related to the behavioral data. Social behavior recordings were taken during one hour daily for five consecutive months; the data were adjusted twice in order to follow the cycles of the females with regular menstruations. By plotting in a matrix the relative frequencies of joins and displacements, the social position of each animal as well as the group's social organization and dynamics were evaluated. A clear tendency towards social rejection (emitting less joins and increasing the amount of displacements) was detected during the premenstrual periods of the dominant female which abruptly changed towards affiliation during menstruation. This observation was detected in all group members no matter their age-sex class or social position during the five months of observation. When the data were analyzed following the mid-ranking female's cycle, no consistent changes were apparent. The importance of social stimuli in the modulation and expression of hormone-related behavior is stressed, as well as the need of using social settings in the experimental analyses of premenstrual mood and behavior disorders.