The circadian variation of basal and postprandial plasma glucose and insulin levels was analyzed in 10 healthy individuals and 10 type-1 diabetic patients treated with a glucose-controlled insulin infusion system. In both groups the postprandial glycemic response to identical low caloric mixed meals ingested at 8.00 am, 1.00 pm, and 6.00 pm increased significantly in the course of the day. In contrast to findings in conventional insulin therapy mean postprandial insulin requirements of the diabetic patients increased from 8.5 +/- 3.0 IU for breakfast to 9.1 +/- 2.0 for lunch and 10.9 +/- 3.0 for dinner (p less than 0.01). Repeated studies with different test meals showed that in the diabetic patients the circadian deterioration of carbohydrate tolerance was reduced after a fibre rich meal with low glycemic effect and insulin requirements. In the healthy subjects basal insulin levels at 8.00 am were 35% higher than at 1 and 6.00 pm (p less than 0.01). Basal insulin requirements of the diabetic patients increased significantly from a night value of 0.7 IU/h to 1.18 IU/h during the early morning and remained constant throughout day time. We conclude that circadian changes in postprandial carbohydrate tolerance are independent from the endogenous rhythmics of basal glucose metabolism. In diabetic patients the circadian pattern of postprandial responses is substantially determined by exogenous factors like diet composition caloric intake and therapeutic regimen.