Neuronal responses of association field 5 during elaboration of defensive conditioned reflex to sound were studied in chronic experiments on cats. Neuronal activity at the reflex elaboration was usually observed in the first 50 ms after sound stimulation, i.e. it was connected with the action of signal stimulus, but not with the display of the conditioned movement. The most essential changes in neuronal responses of the association cortex were seen at the initial stage of conditioning. In some neurons under conditioning the reaction was stabilized, in others a decrease in both the background impulse activity and intensity of the responses was observed, in some cases the response to either of the applied stimuli disappeared. The latter circumstance allows suggesting the transition from polysensory to monosensory specialized neuronal reaction during conditioned reflex elaboration.