Cat kidneys perfused with heparinized blood from heart-lung circuits (Davey & Lockett, 1960; Lockett, 1966) reabsorbed 99% of the filtered water and sodium. By contrast, kidneys similarly isolated but perfused from pump-oxygenator circuits reabsorbed only 96·5% of the altered Na and 98% of the filtered water (Lockett, 1967 a). The excessive leak of Na and water from kidneys perfused from a pump-oxygenator system was corrected by insertion of a heart (perfused through the coronary arteries) into the perfusion circuit. The isolated heart released a substance into the blood stream in response to reduction in atrial filling (Lockett, 1967b); an apparently identical substance was extracted, in very small amount, from heart muscle (Lockett, 1967b; Ilett & Lockett, 1968). The physico-chemical properties of the heart substance were consistent with a steroidal structure and the renal actions were qualitatively similar to those of aldosterone but differed markedly in latency of action.