The effect of oral administration of sodium orthovanadate on hepatic malic enzyme (EC 126.96.36.199) and glucose-6-phosphate dehydrogenase (EC 188.8.131.52) activities was investigated in nondiabetic and diabetic rats. Streptozotocin-induced diabetic rats were characterized by 4.7-fold increase in plasma glucose and 82% decrease in plasma insulin levels. The activities of hepatic malic enzyme and glucose-6-phosphate dehydrogenase were also diminished (P less than 0.001). Vanadate treatment in diabetic rats led to a significant decrease (P less than 0.001) in plasma glucose levels and to the normalization of enzyme activities, but it did not alter plasma insulin levels. In nondiabetic rats vanadate decreased the plasma insulin level by 64% without altering the enzyme activities. Significant correlation was observed between plasma insulin and hepatic lipogenic enzyme activities in untreated and vanadate-treated rats. Vanadate administration caused a shift to left in this correlation suggesting improvement in insulin sensitivity.