The variation in the follicular phase of the menstrual cycles of seven parous female baboons was studied. The preliminary part of the follicular phase, before oestrus, was longer under conditions of psychosocial stress. Perineal swelling was increased when the normally groupliving female was placed in tactile isolation, and the colour of the swelling changed from bright pink to a pale grey-pink. The latter change was permanently reversed on return to the group. The former was temporarily reversed, but swellings continued to be abnormally large after a long succession of menstrual cycles in which pregnancy was prevented. There was some evidence that the swellings return to normal, after a pregnancy and lactation interval. The implications of these findings are discussed.