24 young, military subjects participated in a ranger training course of 5 days' duration with prolonged, heavy physical exercise and sleep deprivation. The subjects were divided into two groups showing either large calorie deficiency or nearly isocaloric status. An oral glucose tolerance test was performed on the subjects on day 5 on the course and in a control experiment 8 weeks after the course. During the course the subjects with negative calorie balance showed augmented integrated glucose-induced gastric inhibitory polypeptide (GIP) response (p less than 0.05) and the plasma concentration of GIP after glucose stimulation was higher at 90 and 120 min during the course than in the control experiment. On the other hand, the plasma GIP levels of the subjects on isocaloric diet changed similarly during the experimental period and the control period. In another experiment 11 healthy subjects were given a meal after an overnight fast and after 5 days of absolute fasting. The 5-day fast provoked higher postprandial plasma concentration of GIP between 60 and 150 min after meal stimulation, and the integrated meal-stimulated GIP response increased (p less than 0.01) after the prolonged fasting. The subjects in both experiments showed glucose intolerance. The insulinogenic index decreased during the training course, but increased during the prolonged, absolute fasting. The study shows that there is a dietary modulation of the GIP response to nutrient stimulation in healthy man and that the augmented GIP release is not attributable to insulin release.