In seven insulin-deficient (less than 3 mU/l) pancreatectomised dogs, the direct and glucagon-related indirect effects of intraportal insulin infusion (350 microU/kg-min; 12 +/- 1 mU/l) on glucose production were determined. Insulin was infused for 300 min during which time the plasma glucagon concentration was allowed to fall (314 +/- 94 to 180 +/- 63 ng/l) for 150 min before being replaced by an infusion intraportally at 2.6 ng/kg-min (323 +/- 61 ng/l) for the remaining 150 min. Glucose production and gluconeogenesis were determined using arterio-venous difference and tracer techniques. Insulin infusion shut off net hepatic glucose output and caused the plasma glucose, blood glycerol and plasma non-esterified fatty acid levels to fall. It caused the hepatic fractional extraction of alanine (0.41 +/- 0.10 to 0.21 +/- 0.06) and lactate (0.32 +/- 0.09 to 0.04 +/- 0.03) to fall which increased their concentrations. When glucagon was replaced, all of these changes were fully or partly reversed with the exception of the changes in glycerol and nonesterified fatty acids. Indeed, 70% of the fall in hepatic glucose production and virtually 100% of the changes in lactate and alanine metabolism produced by basal insulin infusion were mediated by a fall in glucagon. However, the fall in hepatic uptake of glycerol was unaffected by changes in glucagon and thus gluconeogenesis from this substrate was inhibited by insulin per se probably as a result of reduced lipolysis. The latter effect of insulin may explain the incomplete restoration of hepatic glucose production when hyperglucagonaemia was re-established during insulin infusion.