Male primates exhibit marked elevation of circulating testosterone levels during the early postnatal period. The aim of this project was to test whether experimental manipulation of circulating testosterone levels in male and female infant rhesus monkeys affected development of the external genitalia during the first six months of life. Four groups of infants were studied. Seven control male infants exhibited high circulating testosterone levels during the first three months of life. Seven males were treated with a GnRH agonist (avorelin) from the first week of life onwards, which suppressed the postnatal testosterone surge. Ten control females exhibited low circulating testosterone levels during the early postnatal period. Administration of testosterone to 10 females resulted in high circulating levels in these infants. Fortnightly blood samples and genital measurements were taken from all infants during the first six months of life. Growth of the penis of avorelin-treated males was significantly retarded when compared to control males. Average length of the penis at six months of age was significantly (p = 0.012) smaller for avorelin-treated males (25.2 +/- 2.8 mm) than for control males (37.3 +/- 3.0 mm). Avorelin-treated males attained only around 50% detachment of the prepuce from the glans of the penis, while control males averaged 90% detachment. Treatment of females with testosterone resulted in significant growth of the clitoris in comparison to control females. The growth rates of the penis of control males and clitorides of testosterone-treated females were similar and greatest during the first two months of life. Gain in body weight was not affected by either hormonal manipulation. It is concluded that manipulation of circulating testosterone levels during the early postnatal period affects penile and clitoral development of infant rhesus monkeys. This postnatal period may therefore represent an important stage in penile development in primates.