Thirty-three post-menopausal women were treated with piperazine oestrone sulphate 1.5--3.0 mg daily on a cyclical basis for 3--6 months. Serum luteinising hormone (LH), follicle-stimulating hormone (FSH), oestrone and oestradiol were measured on two occassions prior to starting treatment, and several times during the course of therapy. Initial mean concentrations of oestrone and oestradiol of 59 and 14 pg/ml rose to 528 and 83 pg/ml, respectively, and this was associated with a dose-related suppression of LH (from 57.1 to 50.3 u/l) and more particularly FSH (from 25.8 to 16.5 u/l). There was a highly significant correlation between the decrease in FSH and the oestradiol concentration during treatment, but no correlation with oestrone.