The role of the renin-angiotensin-aldosterone system in the development of tolerance to the diuretic effect of furosemide was investigated in 12 healthy male volunteers. Furosemide in a dose of 40 mg daily for one week had a brisk acute diuretic effect, but did not lead to dehydration, hyponatremia or fall in blood pressure. The reason for this was a reduction in sodium excretion between doses (rebound effect) and a decrease in sensitivity to furosemide from day 1 to day 7. The latter phenomenon is referred to as delayed tolerance to furosemide. Inhibition of angiotensin converting enzyme with lisinopril 20 mg daily did not change the renal furosemide excretion rate, the renal sensitivity to furosemide or the tolerance development. Thus, delayed tolerance to furosemide diuresis was not related to dehydration or activation of the renin-angiotensin-aldosterone system. Other mechanisms, probably intrarenal, will have to be looked for.