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In chronic experiments with glass microelectrodes responses of 288 spontaneously active neurons in the auditory cortex were investigated in cats at rest (123 neurons) and after defensive conditioning to sound (165 neurons). In the first situation 43% of neurons did not respond to acoustic stimulation. Most (about 60%) responses of the reacting neurons showed marked inhibition. Conditioning caused an increase (up to 72%) in the number of neurons responding to acoustic stimulation, the appearance of tonic responses, a severalfold increase in the amplitude of the responses, an increase in the number of responses of activation type, and stabilization of their form. The results point to increased excitability of neurons in this cortical area.