Single unit spike trains were recorded in the auditory cortex (n = 78) and in the auditory thalamus (n = 55) of nitrous oxide anesthetized cats. The electrophysiological activity was studied before and during the application of pentobarbital (P, 7 mg/kg), ketamine (K, 12 mg/kg) and a mixture of these anesthetics (KP). The units were characterized during the spontaneous and acoustically driven activity ('white' noise and pure tone bursts). For the majority of cortical (61%) and thalamic (83%) units both drugs tended to decrease the spontaneous firing rate, but affected differently its time structure: P tended to increase the average size of burst discharges, whereas K and KP tended to decrease it. In the cortex the peak firing rate evoked by 'white' noise tended to be decreased, whereas stronger excitatory responses were observed in the thalamus after injection of K or KP. The overall effect of the anesthetics during stimulation by pure tones was an increase in tonal selectivity due to a decrease in the response bandwidth. The response pattern to tones was also sometimes affected by the drugs. The direct evidence reported here for significant alterations of the discharge properties of auditory neurons in the thalamus and cortex resulting from low dose administration of K and/or P emphasizes difficulties in comparing data derived from experiments conducted in various conditions of anesthesia or in the awake state.