AIM: To clarify the mechanism by which insulin resistance develops in obesity, Zucker fatty rats (ZFR) and lean litter mates (ZLR) were temporally subjected to oral glucose tolerance tests (OGTT) at 6 and 15 weeks of age. METHOD: As candidates for causative factors of insulin resistance, plasma leptin, free fatty acids (FFA) and tumor necrosis factor (TNF)-α levels were evaluated. RESULTS: There was no difference in the body weight between the two groups at 6 weeks of age, but ZFR were significantly heavier than ZLR at 15 weeks of age. At 6 weeks of age, blood glucose levels and area under the curve of glucose (AUCg) during OGTT were not significantly different between the two groups, while plasma insulin levels and area under the curve of insulin (AUCi) in the ZFR group were significantly higher than those in the ZLR group. At 15 weeks of age, the blood glucose levels and AUCg as well as plasma insulin levels and AUCi in the ZFR group during OGTT were significantly higher than those in the ZLR group. The ratio of fasting insulin to glucose in the ZFR group was significantly higher than that in the ZLR group at 6 and 15 weeks of age. Peripheral and portal plasma leptin and FFA levels were significantly higher in ZFR than ZLR both at 6 weeks and 15 weeks of age. Meanwhile, at 6 weeks, plasma TNF-α levels and expression of TNF-α protein in subcutaneous and visceral fat tissues were similar in both groups; however at 15 weeks, these were significantly higher in the ZFR group than the ZLR group. CONCLUSION: These results suggest that FFA rather than TNF-α may play an important role in early events involved in the development of insulin resistance and TNF-α accelerates insulin resistance together with FFA in the later stage.