We have reported that the monosodium glutamate (MSG)-induced obese rats develop hypertriglyceridemia due to increment of VLDL. In order to clarify the mechanisms of hypertriglyceridemia in MSG-treated rats, triglyceride secretion rates (TGSR) and postheparin lipolytic activities (PHLA) were studied. MSG (4mg/g body weight/day) was injected subcutaneously to the neonatal Wistar rats for 5 days. Experiments were performed using the rats fasted for 14 hours, 24 weeks after MSG injection. TGSR was calculated from the differences in plasma triglyceride concentrations before and 90 minutes after intravenous injection of Triton WR 1339 (135 mg/rat). Postheparin lipolytic activities were measured in plasma obtained from the rats, 3 minutes after injection of heparin, 50 U/250 g body weight. Higher Lee's indices were observed in the MSG-treated rats. Plasma concentrations of triglycerides were 361.5±50.8mg/dl (control 100.4±12.8mg/dl) and 883.2±180.6mg/dl (control 72.9±12.0mg/dl) in male and female MSG rats, respectively. TGSR was significantly increased in the MSG-treated rats (male: MSG 495.1±71.2, control 233.8±12.8mg/dl/hr, p<0.005; female rats: MSG 672.5±22.5, control 399.7±29.8mg/dl/hr, p<0.001). TGSR showed the positive correlation to plasma concentrations of triglycerides (male rats: r=0.75, p<0.005; female rats: r=0.77, p<0.005). Significantly increased levels of plasma IRI (male rats: 15.13±2.16 vs. 6.08±0.35, μU/ml, p<0.005; female rats: 9.53±1.20 vs. 4.42±0.32μU/ml, p<0.005) correlated positively to TGSR. There were no significant differences in total PHLA. However, increased activities of lipoprotein lipase (5.22±0.23, vs. 3.75±0.36μmol/ml/hr) and decreased activities of hepatic triglyceride lipase (10.16±0.26 vs. 11.12±0.22μmol/ml/hr) were observed in the male MSG-treated rats. The data presented indicated that increased TGSR induced by hyperinsulinemia plays an important role in hypertriglyceridemia observed in the MSG-induced obese rats.