1. Haemodynamic studies were performed in six patients with hypertension, hyper-aldosteronism and low plasma renin (five patients with a solitary adrenocortical adenoma, one patient with bilateral adrenocortical nodular hyperplasia), and in ten normotensive control subjects.
2. Studies in the chronic phase of hypertension uniformly showed elevated total peripheral resistance while cardiac output was not increased.
3. In four patients haemodynamics were studied in the early phase of hypertension following a normotensive period induced by spironolactone. Under these latter conditions the raised blood pressure was associated with increased cardiac output whereas total peripheral resistance was normal. It is suggested that the haemodynamic pattern observed during the phase of the renewed elevation of blood pressure is similar to that at the onset of aldosterone-induced hypertension.
4. Serial measurements in two patients revealed that the haemodynamic characteristics were dependent on the phase of hypertension: during the chronic phase total peripheral resistance was increased whereas cardiac output was not. The new rise in blood pressure following discontinuation of spironolactone therapy was associated with increased cardiac output while total peripheral resistance was normal.
5. Although limited, the findings suggest that the initial step in the development of aldosterone-induced hypertension is a rise in cardiac output. This may be an important factor for the final elevation of total peripheral resistance.