The neural mechanisms involved in the detection of natural complex sounds were studied by recording single-neuron responses from 132 cells in the primary auditory cortex of the cat. The cats were paralyzed and under neuroleptanalgesia (NLA). The cells were first stimulated with pure tones; the responses were then compared with those evoked by many different types of complex sounds, most of which were animal vocalizations. Per-stimulus-time (PST) histograms constructed from the responses to repetitive stimuli were compared with the corresponding sound spectrograms formed from the sounds used as stimuli. Of 100 cells 68 per cent gave predictable responses to complex sounds on the basis of their responses to different pure tone frequencies. In 32 per cent of the cells the responses were unpredictable. Half of these cells did not react to pure tones at all but responded to one or more animal vocalizations or generator sounds with different patterns. Some cells reacted to pure tones in quite a different way than to certain complex sounds, e.g. with inhibition instead of excitation. These results indicate that cells in the primary auditory cortex of the cat reacting in an unpredictable way to sounds with a complex structure have a more or less specialized function, in detecting and analyzing natural and other complex sound patterns. Cells reacting phasically to pure tones seem to be involved in the detection of transient sound elements.