Cells in the cat primary auditory cortex (A1) were investigated to see whether they could integrate sound signals over time. A1 cells responded well to frequency-modulated sweeps. When a portion of the sweep was replaced by silence the response was weakened considerably. However, the response strength was restored when the silent portion was replaced by a burst of band noise, even though the cells did not respond to the burst of noise alone. These results indicate that A1 cells do not respond simply to instantaneous characteristics of acoustic stimuli but respond to those integrated over time.