Models studying in vivo insulin animal action usually employ single-use anesthetized animals, mainly for technical reasons. We developed a modification of the euglycemic insulin clamp technique and used it to repeatedly assess in vivo insulin effects in awake streptozotocin-induced diabetic rats, and in weight- and/or age-matched controls. Permanent catheters implanted into the left carotid artery and the right jugular vein were used for miniature blood sampling (20 microliters) and recycling. Insulin was infused at 1, 2, 3, 15, and 30 mU/kg.min. Plasma insulin and C-peptide levels and glucose utilization rate were measured at blood glucose levels of 100 mg/dl. Diabetes was associated with diminished elevation of plasma C-peptide and insulin levels after ad lib feeding, 50% decreased (p < 0.005) insulin sensitivity, 31% decreased (p < 0.001) insulin responsiveness, and unchanged insulin clearance rates. Thus, using repeated clamps of the same rat over a prolonged period of time, we demonstrate that diabetes is associated with unchanged clearance but decreased sensitivity and responsiveness to insulin.