Krebs-Ringer solution in which pancreatic tissue of normal rats had been previously incubated, did not stimulate the glucose uptake of the isolated rat diaphragm. An increased glucose uptake was observed, however, after adding incubate of pancreatic tissue from rats in which the pancreatic ducts had been ligated 6 weeks prior to the experiment. Glucose uptake in the isolated diaphragm was also increased by joint incubation with pancreatic tissue of normal rats. Diaphragmatic glucose uptake was not stimulated by joint incubation with pancreatic tissue of alloxan-diabetic rats. It was concluded that insulin is released from pancreatic tissue under in vitro conditions. Unless muscular tissue is present, this insulin is completely inactivated, presumably by proteolytic pancreatic enzymes.