In a prospective double-blind study of the effects of two low-dose oral contraceptives (OCs) on lipid and lipoprotein metabolism, two groups of eighteen young healthy women selected at random were submitted to a six months' use of either monophasic ethinylestradiol (EE) + desogestrel (DG) or triphasic EE + levonorgestrel (LNG). Total cholesterol (C), triglycerides (TG), phospholipids (PL), low density lipoprotein-cholesterol (LDL-C) and high density lipoprotein-cholesterol (HDL-C) together with apoproteins A-I, A-II, B and lecithin cholesterol acyltransferase activity (LCAT) were determined in serial plasma samples collected before, at three and six months during, and one month after OC use. Cholesterol and apoproteins (A-I, A-II, B) composition of lipoproteins (HDL-2, HDL-3, LDL) isolated by ultracentrifugation were additionally determined. On Mono-EE + DG, plasma TG (+39.3%, +45.6%), PL (+21.9%, +16.8%) and apo A-I (+35.5%, +23.3%) levels were significantly increased at 3 and 6 months of use; plasma HDL-C (+24%) and Apo A-II (+21.4%) were transiently increased at 3 months. Lipid and apoprotein composition of HDL-2, HDL-3 and LDL were unchanged. On Tri-EE + LNG, a slight but not significant decrease in HDL-C was observed throughout the study while other plasma lipids and apoproteins were unchanged. Ultracentrifugation revealed a lower content of C (-44.2%) and apo A-I (-44.6%) in HDL-2. LCAT activity expressed as molar esterification rate (MER) rose in a more sustained way during EE + DG use than during EE + LNG treatment. Covariance analysis shows a further significant difference between results of both treated groups for the Apo AI/Apo B ratio that was increased by Mono-EE + DG.