Study on a diurnal rhythm of plasma aldosterone (PA) and plasma renin activity (PRA) was performed in 8 patients with congestive heart failure. All patients had been digitalized and received diuretics under mild sodium restriction. An obvious diurnal rhythm of PA similar to the normal subjects, with the lowest value in the evening and the highest value in the morning, was observed in 7 of 8 cases, while a diurnal rhythm of PRA was obscure except in one case. The PA generally did not run parallel with PRA. Although the reason of the absence of PRA diurnal rhythm in congestive heart failure was not clear, it was considered that reninangiotensin system did not play a significant role for the development of PA diurnal rhythm in congestive heart failure. The determined PA values were entirely within normal range except in 2 cases, although they were administered the potent diuretics chronically. A high PA value was observed only in early morning in one case, while all determined PA values were extremely high in another case with severe congestive heart failure involved in cardiac liver cirrhosis. The PRA values were relatively low in 2 cases, normal in 5 and high in one.