By the use of the intraperitoneal injection method, the in vivo effect of various concentrations of insulin or pro-insulin alone and in combination has been studied on the conversion of glucose-U-C-14 into labeled lipid of epididymal fat pad and labeled glycogen of the diaphragm of rats. On a molar basis proinsulin is one eighth as effective as insulin in promoting glucose oxidation in both tissues. Proinsulin exerts no antagonistic or synergistic effect on insulin but exhibits an additive effect with submaximal concentrations of insulin.