Spikes were recorded extracellularly and IPSPs intracellularly from auditory cortical neurons of cats immobilized with D-tubocurarine in response to stimulation of geniculo-cortical fibers. Fibers whose stimulation induces IPSPs in auditory cortical neurons mainly have low thresholds. When two stimuli, each of which separately evoked an IPSP of maximal amplitude, were applied to them the shortest interval at which the second stimulus evoked an effect was 2.5–3 msec. This effect consisted of an increase in the duration of the integral IPSP, the amplitude of which either remained unchanged or increased under these circumstances by only 5–10%. The interval at which a separate IPSP appeared in response to the second stimulus depended on the duration of the ascending phase of the IPSP and varied from 4 to 22 msec for different neurons. The amplitude of the second IPSP in this case depended on the interval between stimuli. Under moderately deep pentobarbital anesthesia the number of neurons responding to stimulation of the geniculo-cortical fibers by spikes fell sharply but the number of neurons responding by primary IPSPs remained almost unchanged. Under very deep pentobarbital anesthesia, when spike responses of the cortical neurons completely disappeared, the IPSPs also were completely suppressed. It is concluded that inhibitory neurons of the auditory cortex are excited by thick low-threshold fibers, they have a short refractory period, and they are resistant to the narcotic action of pentobarbital.