The effects of labetalol on plasma lipoprotein metabolism were evaluated in a 3-month double-blind drug versus placebo study conducted on 30 consenting hypertensive patients, 15 of whom had normal plasma lipid levels and 15, minor type II hyperlipoproteinaemia; 20 patients received labetalol 400 mg/day and 10 the placebo. All patients remained in stable nutritional status throughout the study. Full clinical examination and blood sampling were carried out 30 days before, and on days 0, 30 and 90 of treatment. Whole blood was collected after 12 hours' fasting and immediately centrifuged prior to determination of plasma lipids (total cholesterol and triglycerides, by enzymatic assay), lipoprotein lipids (HDL, HDL2, HDL3, LDL, VLDL separated by ultracentrifugation in density gradient), apoproteins A1 and B (by laser immunonephelometry) and post-heparin lipoprotein lipase activity (PHLA). Significant changes in heart rate and systolic and diastolic blood pressures were noted in patients under labetalol but not in patients under placebo. Lipid and apolipoprotein levels were similar in both groups on day 0, and no significant variation in lipids, lipoprotein lipids and apolipoproteins were observed after 30 and 90 days of treatment with either labetalol or the placebo. At the end of treatment PHLA was unmodified in the group under placebo and raised in the group under labetalol (p = 0.05). The absence of changes in blood lipid values was found both in patients with normal lipidemia and in those with hyperlipidaemia. This study confirms that labetalol in doses of 400 mg/day has notable anti-hypertensive activity and, as previously reported and in contrast with other beta-blocking agents, is devoid of any adverse effect on lipid metabolism.