A study was carried out in 15 adult in-patients with moderately severe hypertension to investigate the efficacy and safety of single and twice daily doses of primidolol (UK-11,443), a new antihypertensive agent with both alpha- and beta-adrenoceptor blocking activity. Patients received placebo for the first 3 days and then the trial drug, at a starting dosage of either 50 mg or 100 mg, for a further 4 days. There were significant decreases in baseline values for some of the cardiovascular parameters from Day 1 to Day 7, and in all the parameters (blood pressures and heart rate, lying and standing) in terms of the values 3 hours post-dose on Days 1 and 7. Probably because of the lowering of baseline values, however, the overall change in the cardiovascular parameters from baseline to 3 hours post-dose was not significantly different when primidolol and placebo were compared. The only side-effects considered to be related to primidolol were mild postural dizziness of short duration (2 patients), mild nausea (1 patient) and orthostatic vertigo (1 patient). No clinically important laboratory abnormalities occurred.