The present study has examined representation of the temporal sequence of sound stimuli in neurons of the primary auditory cortex (AI) in order to investigate how the central auditory system deals with time information. Adult Japanese monkeys (Macaca fuscata) prepared for chronic recordings were used. Stimuli were: noise bursts, tone bursts, and sequentially presented tone bursts. Unit recordings were made from the contralateral AI. During recording sessions, the animal was anesthetized with a mixture of nitrous oxide and oxygen. Post-stimulus-time histograms were analyzed. Results demonstrated that a single neuron in AI often showed totally different temporal firing patterns when different frequencies were presented. Moreover, neurons of this type facilitatively responded to a temporally paired burst of different frequencies with a particular interval ((Delta)t); for a different pair, a different (Delta)t produced a large firing. However, the (Delta)t which evoked a big response did not produce a large excitation when the temporal order was reversed. The interval which elicited the facilitation ranged from ~50 ms up to ~200 ms under the various conditions used. These neurons were sensitive to (Delta)t, frequencies, and the order of presentation, implying that they are capable of representing temporally changing sounds like communication sounds.