The metabolism in plasma of apo(a) and apoB100, the major protein components of lipoprotein(a) [Lp(a)], and the mechanism by which estrogen lowers Lp(a) concentration are both not well understood. Estrogen or placebo were administered to 12 postmenopausal women in a double-blind cross-over design; and after each treatment, apo(a) and apoB100 in Lp(a) were endogenously labeled by i.v. trideuterated leucine. After estrogen treatment, mean Lp(a) concentration decreased during estrogen, from 25 mg/dL, by 20% (P < 0.01); and the mean production rate of apo(a) decreased, from 0.31 nmol/kg.day, by 34% (P = 0.046). In contrast, the mean fractional catabolic rates of apo(a) were similar, 0.36 vs. 0.31/day (P = 0.23). In 6 women, the kinetics of apo(a) and apoB100, the two major proteins of Lp(a), were studied during estrogen and placebo periods. During both periods, the rate of appearance of tracer was similar in Lp(a)-apo(a) and Lp(a)-apoB100, as were the resulting metabolic rates and the changes during estrogen treatment. In conclusion, the findings are more compatible with intracellular synthesis of Lp(a) from nascent apo(a) and apoB100 than extracellular assembly from plasma low-density lipoproteins. Reduced flux into plasma of Lp(a), an atherogenic lipoprotein, could contribute to the lower cardiovascular disease rates in women receiving estrogen replacement therapy.