Changes in plasma concentration of high-density lipoprotein (HDL) cholesterol concentration were measured in healthy young women who received 21-days' oral treatment with either norethisterone acetate (1, 2, or 5 mg daily) or ethinyloestradiol (20 or 50 microgram daily). Similar measurements were also made in groups of women receiving progestogens or oestrogens for gynaecological indications, and in women taking oral contraceptives. Oestrogens alone stimulate increases in HDL-cholesterol, while progestogens alone suppress the concentration. Combinations of oestrogens and progestogens have variable effects depending on the relative doses and chemical compositions of the two hormones. Because HDL-cholesterol concentration is thought to be negatively correlated to cardiovascular disease risks, products which induce a large decrease are to be avoided, though other risk factors must be taken into account in the selection of oral contraceptives.