12 metabolically healthy subjects were i.v. administered equipotent doses (ED30) of tolbutamide (7.5 mg/kg), glisoxepide (0.02 mg/kg) and glibenclamide (0.006 mg/kg). Prior to and 2, 5, 8, 10, 20, 40, 60 and 120 min after the injection the following serum parameters were determined: blood glucose, immunologically measurable insulin (IMI) with the double-antibody method (Hales and Randle) and proinsulin (IMP) enzymatically (ISP-method). The maximum level of insulin follows the injection of tolbutamide with a value of 70.5 micronU/ml after 2 min, of glisoxepide after 5 min (67.0 micronU/ml) and of glibenclamide after 20 min (32.3 micronU/ml). The proinsulin fraction of the total insulin shows a level of 12.5 micronU/ml before the test. After the administration of the three compounds proinsulin increases, too, but reaches only 20-40% of the total immunoreactive insulin. The amount of secreted insulin and proinsulin during the 120-min test is rather the same for the three substances. The average increase of insulin is nearly identical for tolbutamide and glisoxepide, whereas it is less for glibenclamide. Both the mean blood sugar depression and the highest mean increase of proinsulin is reached after glisoxepide. The significance of the one-chain precursor of insulin as a part of the sulfonylurea stimulated total insulin for glucose depression is discussed.