The effect of spironolactone (50 mg b.i.d.) in essential hypertension was studied by measurement of effective renal plasma flow (ERPF), blood urea nitrogen (Ur+), serum creatinine (Cr), cardiac index (CI), plasma volume (PV), body weight (BW), mean arterial blood pressure (MAP), total peripheral resistance index (TPRI), plasma renin activity (PRA) and plasma aldosterone (PA) in two groups of patients. Ten cases had determinations before, after 5 weeks and 4 months of treatment; fourteen cases who had been treated at an average of 18 months, had measurements while on treatment and 5 weeks after cessation of the drug. Among the ten patients ERPF fell in six and increased in four patients during treatment, but was statistically unchanged in the total group. Ur + and Cr were also unchanged by treatment. ERPF was unchanged after withdrawal of the drug. During treatment BW decreased 3.5%, PV decreased in nine and increased in one patient, while PRA and PA increased 426% and 202%, respectively. After cessation of the aldosterone blockade, BW increased 1.9%, PV 10.5% while PRA and PA fell 60% and 48.9%, respectively. MAP fell in eight out of ten patients during treatment. This fall was associated with a fall in CI or TPRI, or both. After withdrawal of the drug, MAP increased in nine and decreased in five of the patients. The data shows that this dosage of spironolactone gave minor adjustments of the systemic and renal circulation in spite of the consistent changes in BW, PV, PRA and PA.