20 non-diabetic and 11 insulin dependent diabetic (IDD) children underwent short time (20 min) bicycle ergometer exercise followed by a 10 min rest period. Glucose, IRI, C-peptide and proinsulin were determined prior to and at the end of the exercise, and again after 10 min rest. While no significant change in mean glucose was observed during exercise in the non-diabetics, significant decreases were observed in IRI, C-peptide and proinsulin. After 10 min rest glucose as well as the three B-cell secretory products increased significantly. The change in glucose was significantly (p less than 0.001) correlated to the change in IRI. In the resting period IRI rose more than C-peptide in some subjects. IRI even rose without simultaneous rise in C-peptide indicating a release of tissue bound IRI. The group of IDD children did not show any significant changes in glucose and total IRI, while the endogenous insulin, as measured by C-peptide, did show a fall during exercise. The same was found for proinsulin. The lack of increased endogenous secretion during the rest period was most likely due to suppression of B-cell due to hyperinsulinism and lack of increased glucose concentrations during the rest period.