To examine the influence of insulin-dependent diabetes on the metabolic response to insulin-like growth factor I (IGF-I), awake chronically catheterized diabetic and nondiabetic BB/w rats received IGF-I (5 micrograms.kg-1.min-1) or insulin (2 mU.kg-1.min-1) for 2 h while maintaining euglycemia. In nondiabetic rats, IGF-I and insulin produced similar twofold increases in glucose uptake, but insulin was more effective in reducing hepatic glucose production (90 +/- 15 vs. 5 +/- 11%; P less than 0.001) and beta-hydroxybutyrate levels (94 +/- 1 vs. 19 +/- 6%; P less than 0.001). In diabetic rats, insulin-stimulated glucose uptake was impaired (8.5 +/- 0.9 vs. 11.5 +/- 0.9 mg.kg-1.min-1 in nondiabetics; P less than 0.05). In contrast, IGF-I-stimulated glucose uptake was identical in diabetic and nondiabetic rats. Furthermore, IGF-I suppressed glucose production by 73% (P less than 0.01) and caused a greater lowering of beta-hydroxybutyrate levels (from 2.9 +/- 0.8 to 0.8 +/- 0.3 mumol/l) in diabetic rats. We conclude that 1) the capacity of IGF-I infusion to stimulate glucose uptake is maintained in spontaneously diabetic BB rats that are insulin resistant, and 2) IGF-I infusion suppresses elevated glucose production rates and plasma ketone concentrations in diabetic rats but is relatively ineffective in nondiabetic rats. Thus the metabolic responses to infused IGF-I do not appear to be diminished in diabetic rats with impaired responses to insulin.