The concentration of FSH in the plasma of guinea-pigs from day 50 of gestation to day 45 of postnatal life was assayed by a radioimmunological procedure utilizing a cross-reaction with the NIAMDD S6 antiserum to rat FSH. At 68 days of gestation the mean plasma FSH concentration of female foetuses was greater than that of the males, although the concentrations in the two sexes were similar on day 50 of gestation. Maternal levels remained consistently low throughout gestation. Postnatally there were no marked changes in plasma FSH levels through to maturity, although a transient rise in the male occurred over the first 3 days after birth. Gonadectomy on days 0, 5, 10, 15, 25 or 35 of postnatal life, or when adult, resulted in a significant increase in plasma levels of FSH within 10 days. The rise in plasma FSH concentration was greater in males than in females at all ages, although, a larger increase was observed in females spayed at 5 or 10 days of age than at other times.