The induction of hyperprolactinaemia in the male rat following chronic high-dose oestrogen administration over 3 months was associated With a significant inhibition of the secretion of growth hormone (OH) (P < 0·02) thyroid stimulating hormone (TSH) (P < 0·0025), luteinizing hormone (LH) and follicle stimulating hormone (FSH) (both P < 0·01). Acute, but not chronic, administration of bromocriptine (1 mg/kg) to these hyperprolactinaemic animals had the effect of normalizing the serum levels of GH and TSH but not those of LH or FSH. While the effects observed on GH, TSH, LH and FSH following induction of hyperprolactinaemia are likely to be consequential to brain actions of prolactin, the present data do not exclude the possibility of direct actions of oestrogen itself.