Treatment with oral prednisone (15 mg every 6 h) for 1 day plus a 4-h glucose infusion at 2.8 mg/min kg body weight to 5 normal, healthy individuals raised their blood glucose to 137 +/- 4.5 mg per 100 ml (mean +/- SEM). In order to evaluate the effects of steroid-induced hyperglycemia on insulin responses, a model for the duplication of blood glucose concentration in serial studies was developed. During glucose infusion at 5.7 mg/min kg body weight, the fractional uptake of glucose at the end of infusion (KG) was 2.08 +/- 0.2%/min and the apparent volume of distribution (V) was 285 +/- 10.5 ml/kg. Further increase in the rate of glucose infusion did not affect KG and V. Based on these parameters, KG and V, the stable blood glucose achieved during the prednisone study (C) was duplicated both after short (4 h) and prolonged (28 h) glucose infusions (138 +/- 4.5 and 146 +/- 4.5 mg/100 ml, respectively) at rates calculated as the product of KG.C.V. The effects of prednisone treatment on insulin secretion were examined (1) during fasting, (2) at identical glucose concentrations during glucose infusions at constant rates, and (3) in response to glucose pulse (0.1 g/kg) during the infusions. During fasting, there was a significant elevation of mean blood glucose with prednisone (99 +/- 1.8 mg/100 ml) compared with that in the control study (88 +/- 1.7 mg/100 ml). The plasma IRI, however, remained unchanged (10 +/- 2.3 vs 10 +/- 1.6 muU/ml). During glucose infusions in the presence of similar blood glucose levels, the IRI was lower after prednisone treatment (18 +/- 1.5 muU/ml) than during the short and prolonged glucose infusions (42 +/- 5.1 and 63 +/- 7.0 muU/ml). The insulin response to the glucose pulse also was significantly lower during steroid treatment. Thus, prednisone apparently has an early inhibitory effect on the insulin response to glucose.