1. The distribution of binaural responses within the ferret's primary auditory cortex was determined by standard microelectrode mapping techniques. Single and multiple unit responses were recorded from the middle ectosylvian gyrus of barbiturate-anesthetized animals with tungsten microelectrodes (1.2-1.8 M omega) inserted into the brain perpendicular to the cortical surface. The characteristic frequency (CF) and binaural response characteristics were determined for each point sampled. 2. Gated pure tones were delivered independently to the two ears through a sealed sound system, and binaural response types were determined by comparison of evoked activity for monaural and binaural stimulation. Most binaural responses fell into one of three major groups: binaural summation (EE/F), binaural suppression (EO/I), or mixed binaural summation and suppression. 3. The neurons tended to be grouped together on the basis of their binaural response properties. Zones of binaural summation and suppression extended across the surface of the middle ectosylvian gyrus and intersected with regions of sound frequency representation. 4. Particular attention was paid to the distribution of binaural responses within isofrequency contours in the ferret's primary auditory cortex (AI). Along the length of each isofrequency contour, areas of EE/F alternated with areas of EO/I. The summation areas were typically between 0.5 and 0.7 mm wide. Single neurons with mixed binaural response properties were frequently found between groups of EE/F and EO/I cells. The mixed responses appeared to mark a transition in location between zones of summation and suppression responses. 5. The distribution of interaural intensity difference (IID) thresholds was also examined along the length of isofrequency contours. No systematic relation was found between IID threshold and the distance along an isofrequency contour.