SUMMARY Plasma human growth hormone (HGH) and serum phosphorus concentrations were measured during the fasting ambulatory state in middleaged men, pre- and postmenopausal women and postmenopausal women who had been taking 20–40 μg mestranol daily for 1–3 years. The mean plasma HGH concentrations were consistently higher in the women than they were in the men, there was little difference between the mean values for pre- and postmenopausal women, and the mestranoltreated women had significantly higher mean values than the untreated postmenopausal women. The mean serum phosphorus concentration was significantly higher after menopause and was significantly lower in those women on long-term lowdose mestranol therapy. A significant direct correlation was found between serum phosphorus and plasma HGH concentrations in untreated postmenopausal women. It is suggested that the postmenopausal relative hyperphosphataemia is consistent with increased HGH activity.