Although the phenomenon of conditioned hypoglycemia is now well established, little is known about its mechanism. The present experiments lead to the conclusion that the mechanism involves a release of insulin: Experiment 1 showed that rats given an injection of streptozotocin, a drug which destroys the beta cells of the islets of Langerhans, did not show a conditioned hypoglycemia, whereas rats given only the vehicle for the streptozotocin did; and in experiment 2, blood drawn from conditioned rats just before the conditioned hypoglycemia would normally occur showed greater insulinlike activity than blood drawn from control rats. These results demonstrate a conditioned release of some hypoglycemic agent that depends upon the integrity of the beta cells. A neural control over release of insulin must be inferred to explain them. © 1970, SAGE Publications. All rights reserved.