1. The hypotensive effect of propranolol and its correlation with the changes in heart rate, plasma volume and plasma renin activity produced by this drug were studied in normotensive rats and in rats with spontaneous, renovascular and deoxycorticosterone-induced hypertension.
2. No correlation was found between the fall in arterial pressure and the reduction in heart rate and plasma volume. On the contrary, a direct correlation was found between the hypotensive response and plasma renin activity: rats with high renin showed the maximum response, those with low renin no response.
3. Studies on blood volume distribution in different organs and the direct measurement of blood flow in the splanchnic bed demonstrated a significant correlation between the effect of propranolol on capillary permeability and the haemodynamic changes which it causes, namely a significant increase in portal venous pressure.