An analysis of data from a prospective study which compared the effect of conjugated equine oestrogens (Premarin) with that of piperazine oestrone sulphate (Ogen) demonstrated that postmenopausal women who received conjugated equine oestrogens had a higher rate of a rise in blood pressure and development of hypertension than women who received piperazine oestrone sulphate. Conversely, a greater proportion of women who took oestrone sulphate had a fall in blood pressure than those who took equine oestrogens. A hypothesis to account for these differences, based on the known potent biological action of equine oestrogen, is developed.