The functional organization of visual cortical area 17/18 of the ferret was studied using a double-label 2-deoxyglucose (2-DG) method. Animals were stimulated sequentially with moving gratings of 2 different orientations. Elongated orientational patches running roughly at a right angle into the area 17/18 border were seen. The orientation maps were similar in areas 17 and 18. When animals were stimulated with 2 orthogonally oriented gratings, activation patterns were not fully complementary but did not overlap. A complex pattern of partial overlap was observed, with orientations differing by 45 degrees. More gradual and more abrupt changes alternated frequently on a scale smaller than the average periodicity in the patterns. A cortical patch representing a given orientation was regularly surrounded by both neighboring and orthogonal orientations. The direction of the orientational changes reversed frequently in the immediate vicinity of any orientational patch. Orientation maps were compared with complete maps of retino- cortical projections obtained by transneuronal labeling with intravitreally injected tritiated proline. Ocular dominance in the binocular segment of both areas 17 and 18 was found to be organized into elongated ipsilateral islands in an almost contiguous contralateral projection. In area 18, the patches were wider than in area 17, and the ipsilateral islands were smaller in the regions representing the upper and central visual field than in those representing the lower visual field.