The spatio-temporal pattern of sound-evoked neural activity in the guinea pig auditory cortex was studied by optical recording with the aid of voltage-sensitive dye. Changes in light intensity induced by sounds at various frequencies and pressure levels were recorded with a 12 x 12 array of photodiodes. The amplitudes of the responses were displayed as sequential two-dimensional images. Tonotopical organization was found in two subdivisions of the auditory cortex, the anterior field (field A) and the dorsocaudal field (field DC). The frequency gradients in fields A and DC had a mirror-image relationship. This agrees with results obtained by the microelectrode technique. However, the tonotopic response observed in our study was transient. The focal activity that began in field A propagated in two directions; dorsally along the iso-frequency bands in field A, and caudally toward field DC. This suggests that the sound information processing initiates at field A, and its outputs are transferred to field DC, which is probably a hierarchically higher center.