This study was performed to determine whether metabolic and hormonal responses during moderate exercise differ between continuous intraperitoneal insulin infusion (CIPII) and continuous subcutaneous insulin infusion (CSII). In seven Type 1 diabetic patients, treatment was changed from CSII to CIPII. Prior to the change, these patients performed an ergometer exercise at 60% of VO2max for 40 min followed by a 200-min rest. About one year later, when the procedure was repeated during CIPII, HbA1c had improved from 8.5 to 7.1%. Arterial blood glucose, venous lactate and hormonal responses were analysed. Although a regimen with a higher basal insulin infusion rate was applied during the exercise test on CIPII, corresponding venous insulin levels were lower (28.0 +/- 2.2 vs. 48.1 +/- 7.9 pmol L-1, p = 0.04). Exercise caused a more marked decline in blood glucose during CIPII, with nadir blood glucose at the end of exercise (3.6 +/- 0.4 vs. 5.1 +/- 0.4 mmol L-1, p = 0.005). Both exercise tests yielded significant and similar increases in plasma levels of adrenaline, noradrenaline, cortisol and growth hormone. A significant rise in plasma glucagon (15.1 +/- 4.5 pg mL-1, p = 0.01) was observed during CIPII, but not during CSII (7.4 +/- 3.5, pg mL-1, n.s.). It is concluded that patients on CIPII should reduce their insulin infusion rate during exercise. CIPII appears to have favourable effects on counterregulatory capacity; in particular, a more prominent glucagon response to exercise may prove important.