The effects of oral dilevalol (an R, R-isomer of labetalol), a new beta-adrenoceptor blocker with beta 2-receptor stimulating and alpha-recepter blocking properties on blood pressure, renal function, plasma renin activity (PRA) and plasma aldosterone have been studied in 15 patients with mild-to-moderate essential hypertension treated with it for 6 weeks. Two patients with apparent treatment failure and one patient who developed muscle pain and cramps, and had an elevated creatine phosphokinase level, were excluded from the study. Dilevalol monotherapy 100 mg once daily for 6 weeks significantly lowered both the systolic and diastolic blood pressure compared to placebo. Total renal vascular resistance was significantly reduced, and RBF and GFR remained unchanged. Dilevalol significantly decreased PRA. The results suggest that prolonged daily treatment with dilevalol preserves renal function and produces a concomitant hypotensive action in patients with mild-to-moderate essential hypertension. The ancillary pharmacological properties of dilevalol rather than PRA suppression may be relevant to its renal effects.