The effect of enalapril alone and in combination with nadolol on resting and exercising blood pressure, heart rate, plasma renin activity, aldosterone, noradrenaline, and potassium levels was studied in 10 hypertensive patients (diastolic blood pressure between 95 and 114 mmHg). Patients received placebo for 4 weeks, enalapril (mean daily dose 24.5 mg) for 8 weeks, and nadolol, 40 mg once daily, was added for the remaining 8 weeks of the study. Exercise testing (modified Bruce, treadmill) was conducted at the end of the placebo run-in phase and at the end of each treatment period. Enalapril reduced resting and exercising blood pressure independent of any change in heart rate: This effect was increased with combination treatment. Plasma renin activity at rest and during exercise was increased by enalapril. Combination treatment attenuated this response and significantly reduced plasma aldosterone. Neither treatment had any effect on plasma noradrenaline levels. Resting plasma potassium levels were increased with combination treatment, whereas both treatment regimens increased plasma potassium levels during exercise. There were no clinically relevant episodes of hyperkalemia. Further investigation is required to qualify the nature of the blood pressure and plasma potassium response with combination treatment.