Plasma free insulin was measured repeatedly for 4 hours following a standardized breakfast in 20 C-peptide negative chronically pumped type I diabetic patients and 5 normal subjects. In the former group, insulin was given as a 1 u/h basal infusion and a 1h superimposed meal-dose of 6 u via a peritoneal (IP) catheter lying in the low (n = 10), or in the mid-abdomen (n = 10). The results of the IP patients were correlated with glycosylated haemoglobin and home capillary blood glucose. Fasting free insulin of IP patients was lower than those of normals (14.7 +/- 0.5 vs 21.0 +/- 1.3 mU/l, p less than 0.01). Dose-induced peak occurred similarly in IP patients and normals (70 +/- 6 vs 70 +/- 12 min.). Values tended to baseline after 165 +/- 15 and 185 +/- 22 min. in IP patients and normals (NS). Results of the mid--and low peritoneum subgroups differed only for peak values (31.5 +/- 2.9 vs 25.0 +/- 1.6 mU/l respectively) and did not correlate with diabetic control.