The reliable detection of reproductive events in free-ranging primates is usually not feasible because of the difficulty of obtaining physiological measures in the field. Utilizing a non-invasive technique, urine was collected from wild Vervet monkeys in Amboseli National Park, Kenya. Measurement of pregnanediol-3α-glucuronide levels in urine of female Vervets, combined with behavioural observations, permitted accurate estimation of conception dates for individual females. Copulatory behaviour in Amboseli Vervets was unrelated to female hormonal cycles, and females copulated both before the initiation of ovarian function and during pregnancy. This study establishes new possibilities for integrative studies of the physiology and ecology of mammals in their natural habitats.