The influence of exercise on glycaemia in the post-prandial state was studied for the first time in non-insulin-dependent diabetic (NIDDM) patients. Meal-induced glucose responses were followed for 8 h in 9 diet-treated patients with NIDDM. Subjects consumed a standardized breakfast and 4 h later a standardized lunch. They were studied in the resting state (control day (CD)) and on another day 45 min of bicycle exercise (53 ± 2 % V˙O 2 max (mean ± SEM)) was performed 45 min after breakfast (exercise day (ED)). On day 3 (diet day (DD)), the breakfast meal was reduced corresponding to the extra energy expenditure during the exercise period on ED. Responses were calculated as areas under the plasma concentration curve (AUC) during 4 h after either breakfast (B-AUC) or lunch (L-AUC). B-AUC for glucose was identical on ED (215 ± 63 mmol/l · 240 min) and DD (219 ± 60 mmol/l · 240 min) and on these days lower (p < 0.05) than on CD (453 ± 78 mmol/l · 240 min). L-AUC for glucose on CD, ED and DD did not differ significantly. B-AUCs for both insulin and C-peptide were also significantly lower on ED and DD as compared to CD (Insulin: 31337 ± 8682, 26092 ± 6457 and 47649 ± 15046 mmol/l · 240 min, respectively. C-peptide: 99 ± 19, 104 ± 26 and 195 ± 31 pmol/ml · 240 min, respectively). Rate of appearance (Ra) for glucose was unaffected by exercise whereas rate of disappearance (Rd) increased significantly. No differences in Ra or Rd were observed after lunch. In conclusion, postprandial exercise of moderate intensity decreases glycaemia and plasma insulin levels after breakfast in NIDDM patients, but this effect does not persist during and after the following lunch meal. Reduction of breakfast caloric intake has the same effect on postprandial glycaemia and insulin secretion as an equivalent exercise-induced increase in caloric expenditure. [Diabetologia (1997) 40: 447–453]