Ovariectomy of prepubertal rats (9 days of age) eliminates the ability of the opiate peptide FK 33-824 to inhibit LH secretion when tested 19 days later. We have investigated whether this removal of opiate inhibition would modify the LH/FSH response to stimulation with oestradiol benzoate/progesterone priming. Ovariectomy of rats during infancy (9 days after birth) amplifies the stimulatory effects of these steroids on LH/FSH secretion when tested 19 days later. This amplification was not seen in rats ovariectomized before (day 24) or after puberty (day 43) and tested 19 days later. The pituitary content of LH/FSH does not appear to contribute to this phenomenon, though increased responsiveness to injected gonadotrophin-releasing hormone (GnRH) is clearly involved; ovariectomy at day 9 is considerably more effective than ovariectomy at day 24 of life in enhancing the response to GnRH. We conclude that infantile ovariectomy either removes, or prevents the development of, a hypothalamic inhibitory mechanism which normally modulates the responsiveness of the pituitary to stimulation with GnRH.