There are circadian rhythms for urinary excretion of Na* and K+in hypertensive patients. In the present study, the changes in the urinary electrolyte excretion pattern and in daily excretion as a function of the time of drug administration have been investigated after separate administration of 3 diuretics at 8 A.M., 2 P.M. and 8 P.M. A dissociation between the natriuretic and kaliuretic effects was observed. Natriuresis was maximum when hydrochlorothiazide and furosemide were administered in the morning, while the potassium sparing effect of spironolactone was greater following its administration in the afternoon. These results can be explained from the time course of the circadian rhythms in renal Na+and K+excretion.