The efficacy and tolerability of the new ACE-inhibitor enlalapril (MK 421) and the beta 1-selective adrenoceptor blocker atenolol for the treatment of primary hypertension were compared in a double blind parallel study. 12 patients were randomized to each drug. The doses of enalapril were 20 and 40 mg o.d and of atenolol 50 and 100 mg o.d. for 4 weeks each, whereafter hydrochlorothiazide (HCTZ) 25 or 50 mg o.d. was added if necessary to achieve a supine diastolic blood pressure (BP) <90 mm Hg 24 hours after drug intake. Supine BP was reduced from 160±7/111±4 mm Hg to 153±13/101±9 mm Hg (p<0.05/p<0.005) with enalapril and from 163±17/109±6 mm Hg to 145±11/95±7 mm Hg (p<0.005/p<0.001) with atenolol. The addition of HCTZ caused a profound additive BP reduction to 132±7/88±6 mm Hg with enalapril and to 130±10/88±7 mm Hg with atenolol. There was no significant difference between the efficacy of enalapril and atenolol alone or combined with HTCZ. The reduction in mean arterial pressure with enalapril tended to correlate with pre-treatment stimulated plasma renin activity and 24 hours urinary kallikrein excretion. Both drugs tended to reduce serum and urinary aldosterone and kallikrein excretion to the same extent. There was one drop-out in each group, one due to impotence on the combination of enalapril and HCTZ and one due to peripheral coldness during atenolol treatment. Other side effects were mild. No toxic adverse effects were registered.