OBJECTIVE To determine mechanisms contributing to the altered lipoprotein profile associated with aging and menopause, apolipoprotein B-100 (apoB-100) and apoA-I kinetic behavior was assessed. RESULTS Eight premenopausal (25+/-3 years) and 16 postmenopausal (65+/-6 years) women consumed for 6 weeks a standardized Western diet, at the end of which a primed-constant infusion of deuterated leucine was administered in the fed state to determine the kinetic behavior of triglyceride-rich lipoprotein (TRL), intermediate-density lipoprotein (IDL), and low-density lipoprotein (LDL) apoB-100, and high-density lipoprotein (HDL) apoA-I. Data were fit to a multicompartmental model using SAAM II to calculate fractional catabolic rate (FCR) and production rate (PR). Total cholesterol, LDL cholesterol (LDL-C), TRL-C, and triglyceride levels were higher (50%, 55%, 130%, and 232%, respectively) in the postmenopausal compared with the premenopausal women, whereas HDL-C levels were similar. Plasma TRL, IDL, and LDL-apoB-100 levels and pool sizes (PS) were significantly higher in the postmenopausal than premenopausal women. These differences were accounted for by lower TRL, IDL, and LDL apoB-100 FCR (P<0.05), with no difference in PR. There was no significant difference between groups in HDL-C levels or apoA-I kinetic parameters. Plasma TRL-C concentrations were negatively correlated with TRL apoB-100 FCR (r=-0.46; P<0.05) and positively correlated with PR (r=0.62; P<0.01). Plasma LDL-C concentrations were negatively correlated with LDL apoB-100 FCR (r=-0.70; P<0.001) but not PR. CONCLUSIONS The mechanism for the increase in TRL and LDL apoB-100 PS observed in the postmenopausal women was determined predominantly by decreased TRL and LDL catabolism rather than increased production. No differences were observed in HDL apoA-I kinetics between groups.