The development of patterns of sexual behaviour of feral-reared subadult, male rhesus monkeys was studied during the time of puberty. Somatic indices of maturation including body weight, dentition, colour of sexual skin, and size of Maturation testes were used to estimate the ages of males. There was a conspicuous change in behaviour after the first ever ejaculation. Prior to this males were poorly oriented towards females and there were very high numbers of un-intromitted thrusts. In the months after the first ejaculation, the sexual performance of subadults remained well below that of adults.