Using the decay phase of the glucose response during an intravenous tolbutamide test, a minimal model of glucose dynamics was used to calculate a value for an "index of insulin sensitivity". This index describes the efficiency of insulin in accelerating the instantaneous rate of glucose disposal, and provides a measure of insulin resistance. The validity of estimates of the index of insulin sensitivity obtained from the intravenous tolbutamide test have been assessed with reference to estimates of this index derived from the intravenous glucose tolerance test for which the model was originally designed. There were three studies: (A) estimates of the index of insulin sensitivity obtained from the intravenous tolbutamide test in a group of normal, healthy men and women were compared with results obtained in a comparable group of subjects using the intravenous glucose tolerance test. The two methods gave estimates of the index of insulin sensitivity that were identical; (B) A group of patients taking methandienone, an anabolic steroid previously shown to cause marked insulin resistance, were tested whilst taking the steroid and either before, or at least two months after treatment. Each patient was tested by both intravenous tolbutamide test and intravenous glucose tolerance test on both occasions. Estimates of the index of insulin sensitivity from intravenous glucose tolerance or intravenous tolbutamide procedures both on and off treatment were significantly correlated (off treatment: rs = 0.71, n = 9, p less than 0.05; on treatment: rs = 0.69, n = 9, p less than 0.05); (C) A group of patient undergoing investigations for suspected disturbances in carbohydrate metabolism was studied, each patient having had both an intravenous tolbutamide and intravenous glucose tolerance test.(ABSTRACT TRUNCATED AT 250 WORDS)