The plasma concentration of oestrone sulphate has been determined in 47 normal postmanopausal women. Oestrone sulphate was measured by radioimmunoassay of the unconjugated oestrone released following enzymatic hydrolysis of the plasma sample. Procedural losses were corrected by the addition of tritiated oestrone sulphate as an internal recovery standard. The mean oestrone sulphate concentration was 295 ± 174 pg/ml with a range from 89–917 pg/ml. In four subjects studied we were unable to demonstrate any diurnal variation in the oestrone sulphate concentrations. We did not find any fall with age and present evidence that the ovary in the postmenopausal woman does not contribute to the circulating level of oestrone sulphate. The most significant observation was the correlation between the plasma oestrone sulphate concentration and the subjects weight ( r = 0.59, P = < 0.001, n = 35). or the degree of obesity.