It has been shown that inhibition of lipolysis and ketogenesis by insulin is more sensitive than suppression of hepatic glucose production and stimulation of tissue glucose uptake. Clinically, on the other hand, blood glucose concentrations fall much more quickly than blood ketone concentrations. Therefore, during continuous insulin infusion at a rate of 3.0 mU/kg B.W./min over a period of 150 min we monitored blood glucose and 3-hydroxybutyrate (3-OHBA) concentrations and muscle 3-OHBA levels using the microdialysis technique in streptozotocin-induced diabetic rats. Blood glucose concentrations decreased rapidly after insulin infusion. However, significant reduction in blood-3-OHBA concentration was found for the last 30 min. In muscle dialysate 3-OHBA concentrations and muscle local blood flow there was no significant change throughout the study. These results suggest that one possible mechanism of delayed improvement in ketonemia is a reduction in muscle removal capacity.