Diurnal variation of urinary potassium excretion was examined on 10 adult patients with chronic renal insufficiency and 24 control subjects without cardiorenal functional disturbances. The daily cycle of potassium excretion, similar to that in controls, was present but less obvious in chronic renal insufficiency. In the controls, urinary potassium excretion reached a peak in the early morning (5.30 to 7.30) with a minimum in the night (21.00 to 5.30). The morning rise in potassium excretion was associated with increases in urinary sodium excretion, arterial blood pH, and plasma potassium and aldosterone concentrations. Creatinine clearance rate did not change significantly. In renal insufficiency, plasma potassium level and potassium excretion increased in the morning as compared with the night, whereas sodium excretion, the creatinine clearance rate, arterial blood pH and plasma aldosterone concentration did not change. 9 alpha-Fluorohydrocortisone gave rise to markedly enhanced potassium excretion. The findings suggest that the blunted cycle of potassium excretion in renal insufficiency is due, in part, to suppressed diurnal variations of sodium delivery to the distal tubule, acid-base balance and/or aldosterone secretion.