We investigated whether the difference of insulin effect existed or not when insulin was administered at different times; just before the light phase (am 6:00) (L-NPH-DM) or just before the dark phase (pm 6:00) (D-NPH-DM) in streptozotocin (60 mg/kg, ip)-induced diabetic rats. Following results were obtained 7 days after STZ administration. Body weight: A significant increase compared to (D-NPH-DM) group was observed in (L-NPH-DM) group. Food intake: Food intake gradually increased after administration of STZ. A significant decrease compared to (saline-DM) group was observed only in the (L-NPH-DM) group. Total lipid content in the liver: A significant increase compared to (saline-control) group was seen in both the (D-saline-DM) and (L-saline-DM) group. In addition, a significant decrease compared to (D-NPH-DM) was observed in the (L-NPH-DM) group. Changes in blood glucose: Different patterns in decrease of blood glucose was observed between (D-NPH-DM) and (L-NPH-DM) groups. In conclusion, the present study demonstrates for the first time that there are differences of insulin effects on body weight, food intake, total lipid content in the liver and blood glucose level with insulin administration time.