The chronic effect of insulin hypoglycemia on the C.N.S. was studied by means of conditioned reactions on 40 rats. Normal rats which jump from a compartment A across a small partition to a compartment B in a response to an electric shock applied to the grid of the compartment (unconditioned response) were trained to react in a similar way in response to a. conditioned stimulus (bell). This reaction was inhibited by lack of reenforcement. Under such conditions no spontaneous recovery occurred but insulin hypoglycemia restored the inhibited conditioned response. The action of insulin was cumulative and might lead to a permanent recovery. This effect of insulin depends upon the number of insulin adms. and the degree of hypoglycemia, coma being the most effective procedure. The reaction is specific since no positive responses are elicited when stimuli are used to which the animal had not been conditioned previously. The chronic effects of insulin coma are not restricted to disinhibition of conditioned responses but influence also the excitatory processes which are the basis of conditioning. This is shown by the fact that a partial conditioning leading to an avg. of only 20% positive responses in the control group causes 82% conditioned responses in the exptl. group subjected to 2 insulin hypo-glycemias during the training period.