The antihypertensive effects of tertatolol, a new non-cardioselective beta-blocking drug, were investigated in 20 patients with mild to moderate primary arterial hypertension, in a placebo controlled double blind randomized study. After tertatolol 5 mg o.d. significant decreases in both systolic and diastolic blood pressure and in heart rate were observed at rest (BP from 155/103 +/- 3/1 to 139/91 +/- 4/3 mmHg p less than 0.01; HR from 79 +/- 2 to 60 +/- 2 bpm p less than 0.01). Peak blood pressure, heart rate and myocardial O2 consumption, indirectly measured as cardiac workload, determined during adrenergic stimulation by 70 degrees head-up tilt, cold pressor test, mental arithmetic stress, isometric exercise and bicycle exercise were also reduced by 4 weeks of tertatolol treatment in comparison to pretreatment levels. No significant changes in the same parameters were induced by placebo. No side effects were observed during treatment.