The structure of orientation maps computed from a different number of stimulus orientations was studied in visual cortical area 18 of the cat. Single condition maps (SCMs) were obtained to 16 stimulus orientations, of which angle maps were generated using 4, 8, and 16 SCMs corresponding to multiples of 45, 22.5, and 11.25 degrees, respectively. The overall orientation distribution of the three types of maps was compared on a pixel-by-pixel basis. Twenty percent of the pixels of the 4-orientations maps differed by more than +/-17 degrees from those produced by 16 orientations. Maps of 8 orientations differed by 6.4 and 5.8% from those of 4 and 16 orientations, respectively. Structural differences between the maps were mainly found at locations displaying high rate of change in orientation preference, i.e., orientation centers and adjoining short, fracture-like zones. These changes included lateral shifts up to 155 microm (average: 38.7 microm) in the position of orientation centers and appearance/disappearance of orientation centers when compared between different conditions. In general, these changes were three times more frequent between maps of 4/8 and 4/16 orientations than 8/16 orientations. It is concluded that orientation maps should be calculated from activity maps representing 8 or more stimulus orientations.