A compartmental model formed by plasma glucose, proinsulin, insulin, and C-peptide was proposed to allow a quantitative evaluation of the interrelationship among the different components of the system and to obtain a better discrimination between normal and pathologic subjects. In 11 control subjects, in 6 mild diabetics, and in 9 severe diabetics (insulin-dependent), the kinetics of plasma glucose, insulin, and C-peptide after an i.v. injection of glucagon (sampling for about 120 min) were fitted to the model which was solved with digital computing techniques. Since biphasic plasma insulin and C-peptide kinetics were demonstrated in many normals and mild diabetics, the effect of glucagon in the model was represented by a differential plus a proportional effect. The model (formed by 6 compartments and 14 transfer constants) takes into account the fact that insulin and C-peptide derive monomolecularly from proinsulin and that liver inactivates insulin. The values of the parameters obtained were submitted to stepwise discriminant analysis in order to obtain their relative importance in discriminating among the three groups of subjects. With respect to normal subjects, we found in diabetics an increased inflow of glucose into plasma; a decreased effect of glucagon in promoting the proinsulin response; a decreased effect of glucose in promoting the proinsulin response; a decreased glucose utilization; a lower coupling effect of insulin on glucose; and a higher disappearance rate of C-peptide. We observed a lower formation of insulin and C-peptide from proinsulin in severe diabetics.