In the rhesus monkey, the posterior bank of the superior temporal sulcus forms part of the prestriate visual cortex and has two regions, a medial one and a lateral one, which have their own separate callosal connections. The afferent input to these two regions was studied in experiments where the corpus callosum was sectioned, and labelled amino acids were injected into other visual areas. By this method, it was found that area 17 projects to that part of the superior temporal sulcus occupied by the more medial of the two callosal inputs. By contrast, the part of the sulcus occupied by the more lateral callosal input was found to receive a strong projection from the fourth visual complex, an area rich in colour-coded cells. Recordings were made from single cells in the superior temporal sulcus in animals in which the corpus callosum had been sectioned previously. The degeneration produced by this procedure was used to provide anatomical landmarks enabling us to assign cells to the lateral or the medial regions of the sulcus. Such recordings revealed that receptive fields were topographically organized in the lateral part of the sulcus and that most cells were colour specific. By contrast, cells recorded from in the region of the more medial callosal patch within this sulcus were directionally selective, without any obvious colour coding. It was concluded from these combined anatomico-physiological experiments that there are at least two distinct regions in the superior temporal sulcus which have different afferent connections and functional properties.