Thirty-eight patients already treated with atenolol 50 mg once daily were randomly assigned to treatment with either hydrochlorothiazide (12.5-25 mg once daily) or lisinopril (10-20 mg once daily) for 8 weeks in a double-blind crossover study. Eight weeks' treatment with the combination of ACE inhibitor and beta-blocker or the diuretic and beta-blocker produced falls in blood pressure (lying: -8.4 +/- 15.4/ -7.3 +/- 80 mmHg and -6.1 +/- 15.3/ -5.2 +/- 8.8 mmHg [mean +/- SD] for lisinopril and hydrochlorothiazide respectively; standing: -10.2 +/- 14.2/8.2 +/- 9.2 mmHg and -6.8 +/- 14/ -6.3 +/- 10.3 mmHg for lisinopril and hydrochlorothiazide respectively) which were not statistically significantly different. Heart rate was significantly increased on the combination of beta-blocker and diuretic (lying: +4.3 +/- 10.7; standing: +3.2 +/- 10.0 beats/min) compared with a fall on beta-blocker+ACE inhibitor (lying; -0.5 +/- 7.6; standing: -1.5 +/- 7.4). Both therapeutic regimens were equally well tolerated. These results suggest that where patients fail to respond to monotherapy with a beta-blocker the addition of an ACE inhibitor may be as effective as the more traditional option of diuretic therapy.