The possible occurrence of long-term changes in gonadotrophin control mechanisms following the administration of oestrogen to adult female rats has been studied. Administration of 2-5 mg oestradiol benzoate (OB) to normal female rats at 60 days of age did not result in failure of ovulation at 120 days of age but significant impairment of the LH and FSH responses to progesterone after ovariectomy and oestrogen priming was observed at 160-180 days of age. Treatment with the same dose of OB at 60 days of rats injected with 10 microgram testosterone propionate on Day 4 of postnatal life resulted in an increased incidence of failure of ovulation at 120 though not at 150 days of age but did not further impair the already reduced gonadotrophin response to progesterone at 160-180 days of age. Removal of the ovaries at 60 days of age did not modify the effects of oestrogen given at 60 days of age in either group nor did ovariectomy at 60 days improve the response of neonatally androgen-treated rats to progesterone at 160-180 days of age. The increases in plasma prolactin and TSH levels in response to oestrogen priming after ovariectomy were not affected in any of the experimental groups. The administration of a long-acting oestrogen preparation (oestradiol cyclopentyl propionate, 2-5 mg at 60 days of age) to normal female rats suppressed ovulation and depressed plasma LH and FSH concentrations for at least 90 days; anterior pituitary weights were greatly increased and plasma prolactin concentrations were very high.