The menstrual cycle characteristics of 2 groups of adult female Chacma baboons were compared. Out of a group of 27 female baboons, 41% (11/27) failed to cycle during the 6 month study period but were all under the stresses of fairly severe experimental manipulation. Data on the menstrual cycles of the remaining 16 unmanipulated females was collected and carefully analysed. These 16 females were individually caged and denied social contact with other baboons. Their menstrual cycles were then contrasted to those of a group of 8 individually caged females which received 24 minute mating tests with vasectomized males on alternate days during their cycles. The females denied the social contact of the mating tests showed significantly longer cycles as a result of significantly lengthened periods up to the time of perineal detumescence.