Changes in behavior in response to light flashes, the conditional stimulus of a defense reflex, were not observed following the administration of dilantin, a preparation which attenuates posttetanic potentiation, in rabbits with preliminarily developed defense and inhibitory conditioned reflexes; weak disinhibition of movements appeared in response to the action of flashes against the background of a conditional inhibitor. The similarity of the reactions of the neurons of the visual cortex to light flashes, the CS, with the responses to their joint action with the reinforcement, did not weaken in the majority of instances, while in several neurons, it intensified. The differences in the responses to reinforced and inhibitory light flashes did not disappear following the administration of dilantin, but they attenuated in some of the neurons due to filling of the inhibitory pause with impulses. The results obtained make it possible to draw the conclusion that posttetanic potentiation does not participate in the maintenance of developed conditioned reflexes.