Abstract10 mU of NSILA‐S, as determined by the fat pad assay, lowered blood glucose and free fatty acids of adrenalectomized rate for a much longer period than 10 mU of crystalline insulin. NSILA‐S was not measurably inactivated by the liver during a 2 h cyclical perfusion, whereas insulin rapidly lost activity with a half‐life of 42 min. 6 mU of NSILA‐S injected intravenously stimulated the incorporation of [6‐14C] glucose into the diaphragm of streptozotocin‐diabetic rats to a much greater extent than 6 mU of insulin, whereas their effects on the incorporation of carbon 14 into the fat pad were equal.When injected intraperitoneally in maximal doses to normal rats both agents stimulated the incorporation of [U‐14C] glucose into adipose tissue and diaphragm to the same extent. There was no potentiation of either agent by the other on adipose tissuein vitro, indicating that they affect the same transport site of the cell membrane. Insulin and NSILA‐S influenced the metabolism of adipose tissue of fasted‐refed rats in the same manner.It is concluded that i.v. injected NSILA‐S is more effectivein vivothan comparable doses of insulin because it is not inactivated by the liver. Furthermore, it seems to have a particular affinity to receptors of the muscle cell membrane. This may be an additional factor in protecting it from inactivati