In animals with experimentally induced stable hyperglycemia attended by heparin deficiency in the blood, the insulin-heparin complex (the ratio of heparin and insulin in the complex was 1:4.7) produces a stronger hypoglycemic effect than that of an equivalent dose of insulin. A simple mixture of insulin and heparin (in the same ratio) causes an equally strong hypoglycemic effect. In animals with experimentally induced stable hyperglycemia without attendant heparin deficiency in the blood the hyperglycemic effect of the insulin-heparin complex and the mixture of these preparations does not differ from that of an equivalent dose of insulin. The results of the study bear evidence that intensification of the hypoglycemic effect in administration of insulin with heparin is due to the existence of hypoheparinemia, and confirm the concept that for adequate manifestation of the hypoglycemic effect of insulin a sufficient concentration of heparin is necessary.