The responses to tonal stimuli of 383 single neurons in physiologically defined primary auditory cortex (AI) of barbiturate-anesthetized cats were examined using sealed stimulating systems incorporating calibrated probe-microphone assemblies. Frequency-threshold curves were obtained for 196 cells and of these, 191 showed narrow frequency tuning. Sharpness of tuning was dependent on the best frequency (BF) of the units, but not on threshold at BF. Thresholds to BF tones were lowest for cells with BFs in the range 4-12 kHz. In electrode penetrations normal or near normal to the cortex surface, cells had similar or identical BFs; tuning characteristics other than BF were not systematically depth related. Spike count versus intensity functions for BF tones were obtained for 61 cells; 80% of these cells had dynamic ranges of 40 dB of less; 14/16 cells had nonmonotonic intensity functions. Minimum latencies were obtained for 88 cells; latencies were shortest in the middle cortical layers. These data indicate that in all respects examined, the frequency tuning of AI cells reflects faithfully that of thalamic and brain stem auditory neurons. It does not appear that convergence of information at the cortical level is reflected in the frequency-tuning properties of single neurons.