The effect of the administration of oestradiol benzoate and of progesterone on the subsequent response to LHRH has been investigated in women with hyperprolactinaemia. There was an amplification in the release of LH in four out of ten patients and of FSH in one out of ten patients at 44 h after the administration of 2-5 mg oestradiol benzoate. The average amount of LH released before and after oestrogen did not change, but there was a significant decrease in the amount of FSH released. There was no correlation between the LH released and the oestradiol concentration in serum at the time of the LHRH tests but there was a negative correlation between the FSH released and the oestradiol concentration (r = 0-507;P less than 0-05). These results contrast with those obtained in normal subjects in the follicular phase of the cycle when there is a positive correlation of oestrogen concentrations and the amount of LH and FSH released. As in normal subjects, however, a significant suppression of basal FSH concentrations, persisting until 44 h, was produced by the oestrogen (P less than 0-01). Seven out of eleven patients showed an amplification of LH response and six out of eleven an FSH response 20 h after the administration of 25 mg progesterone. The mean amplifications are not significantly different from those of normal subjects tested in the early follicular phase of the cycle, but are significantly less than those tested in the mid follicular phase of the cycle (LH P less than 0-001; FSH P less than 0-01). This may be related to the serum concentrations of oestradiol which in patients with hyperprolactinaemia are significantly less than those found in the mid follicular phase of the cycle (P less than 0-05). These results indicate that in women with hyperprolactinaemia oestrogen negative feedback, necessary for cycle initiation, is normal: failure of ovulation may be related to failure of positive feedback to oestroen. Oestrogen-negative feedback is unopposed and this may explain the follicullar development and lack of oestrogen in the mid-follicular phase.