The effects of 6 weeks of treatment with dilevalol 100 mg once daily, or carteolol 10 mg once daily, on renal blood flow (RBF), glomerular filtration rate (GFR) and total renal vascular resistance (TRR) were studied in 10 patients with mild-to-moderate essential hypertension in a randomised cross-over experiment. Both drugs lowered the systolic and diastolic blood pressures to a similar extent without altering the heart rate. Carteolol non-significantly decreased RBF by 9.2% and GFR by 12.3% without altering. TRR, whereas dilevalol produced a significant reduction in TRR by 13.2% (p less than 0.05), a non-significant decrease in RBF by 4.6% and no change in GFR. Neither drug changed plasma osmotic pressure, serum total protein concentration, electrolytes or plasma aldosterone concentration. Plasma renin activity tended to be lower in the dilevalol phase as compared to the carteolol phase. The results suggest that dilevalol may cause a greater decrease in TRR and less reduction in GFR when compared to carteolol in patients with mild-to-moderate essential hypertension. The difference in the renal effects might be due to the difference in the potency of vasodilatory properties of both drugs at the doses applied.