The sexual behaviour of heterosexual pairs of eight female and six male adult pigtail monkeys, Macaca nemestrina, was observed for a 15-min testing period on alternate days of a complete menstrual cycle. The behaviour was correlated with changes in swelling of the sexual skin and in plasma progesterone. A close relation between the peak of sex skin swelling and the time of ovulation was established by laparotomy. Aggressive behaviour was spasmodic and there was very little display of social patterns of behaviour, such as grooming or pouting. 'Presenting' by the female was significantly more frequent in the follicular phase but the majority of other behaviour patterns, including female 'reaching back', frequency of mounting and intromission by the male and number of pelvic thrusts per intromission, did not differ significantly between follicular and luteal phases of the menstrual cycle. The proportion of tests on which ejaculation occurred rose during the follicular phase and reached a maximum at the time of peak sex skin swelling. During the luteal phase, a rise in ejaculatory latency and a marked decline in the incidence of ejaculation, coincided with detumescence of the female's sex skin and elevation of her plasma progesterone. The reduced incidence of ejaculation in these time-limited tests was accompanied by a decrease in the rate of intromission and rate of pelvic thrusting. The ejaculatory behaviour of the male was sensitive to the hormonal status of the female and was inhibited by the presence of progesterone in her blood.