SUMMARY One group of animals received androgen prenatally, by daily s.c. injections of 5 mg testosterone propionate (TP) to the doe on days 24 to 41 inclusive of pregnancy, while a second group remained untreated. Some animals of each group were gonadectomized on the day of birth (= day 1) or on day 2. Weight at birth served as a measure of prenatal growth. Comparisons were made between animals born in litters of identical size. Postnatal growth was studied by weighing the animals at 5-day intervals until the age of 130 days. Comparisons were made between groups of animals that were matched for their weights at birth. At birth males from TP-treated mothers were heavier than their sisters and heavier than control males. After birth, control males grew at faster rates than control females. This difference was not affected by neonatal gonadectomy, but from pubertal age onward the difference decreased and gonadectomized males and females were similar in growth. Female pseudohermaphrodites (from TP-treated mothers), both intact and neonatally ovariectomized, had growth curves similar to those of appropriate male controls at prepubertal ages; at postpubertal ages they were not significantly different from their female counterparts. It is concluded that the sex difference in the rate of prepubertal growth is determined by the absence or presence of androgens before birth, and that the difference in postpubertal growth rate is caused by the stimulatory action of the testes and inhibitory action of the ovaries. It was further found that normal female guinea-pigs have a secondary spurt in growth rate, some time after puberty, whereas in males this was much less clear.