The different effects on serum lipoprotein(a) [Lp(a)] levels by administering estrogens at different times of the day (8 a.m. and 8 p.m.) were evaluated in twenty-four post-menopausal women. Patients were assigned to one of the two treatment groups by random sampling numbers. Patients of both groups received 0.625 mg conjugated equine estrogens daily per os for 21 days. Group A patients (n: 9) received the pill at 8 a.m., and group B patients (n: 12) received the pill at 8 p.m. There were no statistically significant differences between the two treatment groups as regards the anthropometric characteristics, the basal values of the Lp(a), the sex steroid and pituitary hormone levels. Administration of conjugated equine estrogens resulted in decreased levels of Lp(a) only in group B after treatment. The different Lp(a) behaviour in the two groups and in the presence of the same serum hormonal levels, seems to be dependent on the existence of a circadian rhythm of the hepatic responsiveness to estrogens, whose expression is higher during evening hours.