Twenty patients, aged 30-60 yr, who had undergone bilateral ovariectomy, were treated orally with 5 mg medrogestone (6,17-dimethylpregna-4,6-diene-3,20-dione) and 1.25 mg conjugated oestrogens per day, according to a constant dosage pattern during the cycle (22 + 6 days). The lipids and lipoproteins were determined twice before the start of therapy and 3, 6 and 12 mth thereafter. The lipids were quantified enzymatically and the lipoproteins by quantitative lipoprotein electrophoresis. Whilst cholesterol and triglyceride concentrations showed no detectable change, a slight but significant increase was seen in the high-density alphalipoprotein (HDL) cholesterol concentrations. The low-density beta-lipoprotein (LDL) cholesterol level showed a moderate fall. There was a resultant reduction in the beta/alpha-lipoprotein ratio. Accordingly, the apoprotein A1 concentrations were found to be elevated, while apoprotein B tended to fall to lower levels during therapy. When these changes are measured by the lipid metabolism risk criterion for the occurrence of coronary heart disease applicable to post-menopausal patients, the effects of the above-mentioned combination may be regarded as entirely favourable.