In seven cats the effect of posterior hippocampal stimulation on the number of passings over a transverse bar in a shuttle-box was determined before and after induction of a conditioned avoidance reaction in the same experimental situation. The effect of hippocampal stimulation on the reaction was also studied applying it during the conditioning stimulus. It was observed in these experiments that a behavior imitating the conditioned reaction acquired during training, appeared in most cases during stimulation. It was also found that only strong stimulation of the posterior hippocampus applied together with the conditioning stimulus inhibited the reaction.