Normal rats were immunized with insulin and Freund's adjuvant and submitted to an intravenous glucose tolerance test. Plasma glucose and free and total IRI levels were determined and compared to those observed in untreated rats, and in animals injected with the Freund's adjuvant used for the immunization procedure. In six of the 15 insulin injected animals, a significant amount of IRI (more than 100 mU/l) was found to circulate in bound form. In these animals, the fasting plasma glucose concentrations, and glucose disappearance rates were not different from those observed in all the other groups. However, the rise in their free IRI level was delayed, as was the return to basal level: 45 min after glucose injection, the free IRI concentration was still 98 +/- 29 mU/l in the six immunized rats vs 14 +/- 6 mU/l in those treated with Freund's adjuvant (p less than 0.01). Furthermore, the secondary nadir in the plasma glucose concentration observed at 60 min after glucose injection, was lower in the immunized rats (5.4 +/- 0.5 vs 6.8 +/- 0.3 mmol/l, p less than 0.05). It is concluded that in normal animals, IRI binding in proportions similar to those commonly observed in insulin-treated diabetic patients does not alter glucose tolerance but might lead to abnormal insulin kinetics and secondary hypoglycemia. These results might have implications for the use of closed-loop insulin delivery systems in type 1 (insulin-dependent) diabetic patients with insulin antibodies.