Physiological changes around the time of puberty were studied in a longitudinal manner in male and female stumptail macaques. In the male, testicular descent (mean age 3.3 years) is an early sign of puberty, which coincides with onset of the pubertal spurt in body weight and testis growth and precedes the pubertal rise in plasma testosterone. In the female no clear external signs of puberty have been found. Measurements of plasma progesterone yielded proof of first ovulation (mean age 3.7 years), a late sign of puberty in primates. This event coincided with a fall in the rate of body weight growth at the end of the pubertal growth spurt. Reproductive capacity of both males (first ejaculation) and females (first conception) appears to be reached around the 4th birthday.