Oral administration to five postmenopausal women of dl-norgestrel (0.075 mg/d for 7 wk) reduced mean fasting plasma levels of triglycerides by 29% (P < 0.001), VLDL triglycerides by 39% (P < 0.01), and VLDL apo B by 26% (P < 0.05), while lowering mean total cholesterol by 7% (P < 0.06). To explain these observations the kinetics of VLDL and LDL apo B turnover were studied by injecting autologous 125I-labeled VLDL and 131I-labeled LDL under control conditions and again in the fourth week of a 7-wk course of dl-norgestrel. VLDL apo B pool size fell by an average of 27% (1.2 vs 1.7 mg/kg, P < 0.06) and production of apo B by 18% (18 vs 22 mg/kg per d, P < 0.05) with unchanged fractional catabolic rate. Production of LDL apo B increased 36% with dl-norgestrel (12 vs 9.4 mg/kg per d, P < 0.05), but this was compensated by a 36% increase in fractional catabolic rate of LDL apo B (0.33 vs 0.25 pools/d, P < 0.005), thereby maintaining pool size. Lipoprotein (a) fell by an average of 12% (16 vs 18 mg/dl, P < 0.06). dl-Norgestrel reduced VLDL triglycerides (40 vs 64 mg/dl, P < 0.05), intermediate density lipoprotein cholesterol (14 vs 19 mg/dl, P < 0.02), IDL apo B (5.3 vs 7.2 mg/dl, P < 0.05), and VLDL cholesterol (3.1 vs 5.1 mg/dl, 0.10 > P > 0.05), in parallel with the reductions in VLDL apo B production and pool size. dl-Norgestrel significantly lowered the production rate of VLDL apo B, thereby decreasing plasma VLDL and intermediate density lipoprotein concentrations.