Altered responses to several agonists have been reported in various smooth muscles from experimentally-diabetic animals suggesting a defective contractile process of smooth muscle Recently, decreased smooth muscle calmodulin levels have been reported in streptozotocin-diabetic rats. However, the effectiveness of insulin on the decreased calmoduiin levels in diabetic rats has not been questioned. Therefore, the present study was designed to examine the effect of insulin on smooth muscle calmodulin levels from streptozotocin-diabetic rats. Calmodulin levels of the smooth muscle were measured by a radioimmunoassay technique. Streptozotocin diabetes caused a significant decrease in tissue calmodulin levels of smooth muscles. Insulin therapy for 20 days did not correct the changes in calmodulin levels of rat smooth muscles, although it normalised blood glucose in streptozotocin-diabetic rats. These findings suggest that the altered smooth muscle calmodulin may contribute the defective contractile responses in diabetes and these changes may be resistant to insulin therapy.