Glucose production and utilization and activities of key enzymes involved in liver and muscle glucose metabolism were studied in post-absorptive streptozotocin-diabetic rats after 12 h of severe hyperglycaemia (17.5±0.5 mmol/l) and insulinopenia (5±1 ΜU/ml). Basal glucose production was increased: 36.6±3.0 mg·kg·min⁻¹, vs 24.4±2.5 in controls (p<0.05); liver glycogen concentration was decreased by 40% (p<0.05); liver phosphoenolpyruvate carboxykinase and glucose-6-phosphatase activities were increased by 375 and 156%, respectively (p<0.001 and <0.01). During a euglycaemic clamp at a plasma insulin level of 200 ΜU/ml, glucose production was totally suppressed in controls, but persisted at 20% of basal in diabetic rats. In these rats, glucose production was suppressed at a plasma insulin level of 2500 ΜU/ml. Basal whole body glucose utilization rate, 2-deoxy-1-[³H]-d-glucose ([³H]-2dG) uptake by muscles and muscle glycogen concentrations were similar in both groups, as well as total and active forms of pyruvate dehydrogenase and glycogen synthase activities. During the euglycaemic clamp, the total body glucose utilization rates and [³H]-2dG uptake by muscles were similar in control and diabetic rats at a plasma insulin level of 200 ΜU/ ml, but lower in diabetic rats at a plasma insulin level of 2500 ΜU/ml. We conclude 1) in recent-onset severely insulinopenic rats, an excessive glucose production via gluconeogenesis prevailed, mainly accounting for the concomitant hyperglycaemia. This excess glucose output cannot be attributed to liver insulin resistance: the gluconeogenic pathway is physiologically less sensitive than glycogenolysis to the inhibition by insulin. 2) Glucose utilization was apparently normal under hyperglycaemic conditions and at a lower insulin plateau of the euglycaemic clamp but suboptimal in the presence of maximal insulin concentrations, suggesting an early appearance of peripheral insulin resistance.