The effects of a glucose tolerance test, 1 g/kg bodyweight by vein (GTT), and of pretreatment with aminophylline (AM), 10 mg/kg initially followed by an infusion for 1 h of 10 mg/kg bodyweight, were examined in 10 dogs of bodyweight 12 to 20 kg which had partial pancreatectomy. Three controls were sham-operated. No change in serum glucose, free fatty acids (FFA) and insulin concentrations was found when less than half the pancreas was removed, or after the sham operation. When AM was given before the GTT, serum insulin concentrations rose to almost twice those in a standard GTT. Dogs with less than 50% pancreas showed a deterioration in GTT within 2 weeks of the operation which progressed with time; maintaining the dogs on a high-protein diet improved the GTT. AM infusion improved the GTT and increased insulin secretion.