Twenty-six postmenopausal women who had been on cutaneous oestradiol treatment for 3-6 months were given either 120 micrograms of 1-norgestrel (n = 13) or 300 mg of progesterone (n = 13) sequentially for another 6 months. The concentrations of cholesterol, phospholipids and triglycerides were determined in plasma and in the HDL, HDL2, HDL3, LDL and VLDL fractions before and after one, three and six cycles of progestin treatment. Already after 11 days on 1-norgestrel, the mean HDL cholesterol and the mean HDL phospholipid concentrations were reduced by 15%. The reduction of the HDL-lipids was mainly confined to the HDL2 fraction which was decreased by 25-30%. L-norgestrel also reduced the mean TG concentration both in the VLDL and the combined LDL + HDL fractions. Progesterone gave only minor changes of the plasma lipids and lipoproteins. Reduced HDL, especially HDL2, concentration, as induced by 1-norgestrel, might increase the risk for ischaemic heart disease. Therefore, it seems that, as regards the effects on the lipoproteins, progesterone might be more suitable than the 19-nortestosterone derivative 1-norgestrel for postmenopausal sequential hormonal therapy.