In a randomised, double-blind, cross over trial, 25 patients with mild to moderate primary hypertension were given nifedipine 20-40 mg twice daily and labetalol 200-400 mg twice daily after a 4 week period on placebo, followed by the two drugs in combination. The BP during placebo therapy was 164/108 mmHg supine and 159/110 mmHg standing. After monotherapy with nifedipine for 6 weeks the supine BP was reduced by 18/13 mmHg and the standing BP by 20/12 mmHg; with labetalol the corresponding figures were 26/15 mmHg and 28/21 mmHg, respectively. The combined therapy induced a larger fall in BP, by 36/22 mmHg supine and by 39/24 mmHg standing; in 21 of 23 patients the BP became normal. The heart rate (HR) decreased during labetalol treatment alone and on the combined therapy. With nifedipine alone, the HR was unchanged in the supine position and increased on standing. Nifedipine increased plasma renin activity (PRA) and urinary aldosterone excretion (uA), whereas labetalol reduced both. During combination therapy, PRA and uA remained unchanged. There was a slight fall in HDL-cholesterol during treatment with labetalol alone and in combination with nifedipine. The fasting blood glucose increased slightly during treatment with each of the drugs, but neither caused a change in the concentrations of glycosylated haemoglobin A1, serum insulin, C-peptide, or plasma glucagon. Adverse effects as a rule were well tolerated and were related to the pharmacological effects of the drugs. Only 2 patients left the trial, both during labetalol treatment.