All commercially available combined oral contraceptives have 17-C-alkylated estrogens (ethinylestradiol or mestranol) as their estrogen components. In postmenopausal replacement therapy nonalkylated ‘natural’ estrogens have been shown to induce less adverse effects in lipid metabolism than alkylated estrogens. To see if this difference is valid also when the respective kinds of estrogens are used as components in oral contraceptives, the lipid metabolic effects from two preparations with the same progestogen component (norethisterone acetate) but with nonalkylated (17β-estradiol; Netagen) and alkylated (ethinylestradiol) estrogen (Netasyn) have been evaluated. Phospholipids, cholesterol, and triglycerides have been assessed in serum and in ultracentrifugally isolated lipoprotein fractions. The serum lecithin fatty acid composition has been analyzed by gas-liquid chromatrography. Netasyn increased the triglyceride content of serum and all lipoprotein fractions while Netagen did not. Netasyn caused a redistribution in the 1-position fatty acids of serum lecithin with an increase in palmitic and a concomitant decrease in stearic acid, a pattern associated with the 17-C-alkylation of a steroid and probably its capacity to inhibit liver excretory function. In the present study the preparation with the non-alkylated estrogen component had less adverse lipid metabolic effects than the conventional oral contraceptive.