In acute experiments on adult cats injection of microdoses of insulin (0.025 unit/kg in 0.025 ml physiological saline) into the posterior hypothalamic nucleus caused the blood sugar level to be raised 10 and 70 min after the injection. The change in the blood sugar level correlated with the EEG activation reaction of the posterior hypothalamic nucleus in response to insulin injection, which spread to the lateral hypothalamus. Meanwhile hormonal action on the posterior hypothalamic nucleus following bilateral coagulation of the lateral hypothalamus no longer caused the rise in blood sugar after 10 min. This suggests that the lateral hypothalamus particpates in the formation of hyperglycemic responses within this time interval of insulin action on the posterior hypothalamic nucleus.