Spatio-temporal patterns of binaural interaction in the guinea pig auditory cortex (AC) were observed using optical recording with a 12 x 12 photodiode array and a voltage-sensitive dye. The amplitudes of the sound-induced light signals from the cortex were transformed into sequential two-dimensional images every 0.58 ms. Binaural sound stimuli evoked an excitatory response followed by a strong inhibition, and contralateral stimuli evoked a strong excitatory response followed by a weak inhibition. Ipsilateral sound stimuli evoked a weak response. Binaural stimulation induced two types of ipsilateral inhibition: a fast binaural inhibition which was detected only after the contralateral and ipsilateral responses were subtracted from the binaural responses, and which appeared 12-25 ms after the onset of stimulation, and a slow binaural inhibitory effect which was clearly observed in the binaural responses themselves, appearing 70-95 ms after the onset of stimulation. The fast binaural inhibition was observed in the same area as the contralateral excitatory response. The inhibited area became stronger and more widespread with increasing intensity of ipsilateral stimulation. We did not observe the specialized organization of binaural neurons as electrophysiologically found in the cat AC, in which binaural neurons of the same binaural response type are clustered together and alternate with clusters of other response types.