It is known that the frequency of ejaculation by male rhesus monkeys shows a well-marked annual rhythm under laboratory conditions (Michael & Keverne, 1971). Recent studies have demonstrated that a major component of the sexual behaviour of the male is androgen-dependent (Wilson, Plant & Michael, 1972; Michael, Wilson & Plant, 1973; Phoenix, Slob & Goy, 1973). We have estimated plasma testosterone in a group of fully adult male rhesus monkeys throughout the year, and during this time they were routinely mated with oestrogen-treated, ovariectomized females. Eight adult male rhesus monkeys (7·6–12·3 kg body wt) were obtained from India. They were quarantined in London for periods of 4–10 months before shipment to the U.S.A. Here they were housed in individual cages in an air-conditioned animal room where lighting was rigorously controlled to give 14 h light/day between 06.15 and 20.15 h. Blood was collected from the saphenous vein and plasma testosterone