Many lines of evidence suggest that the visual signals relayed through the magnocellular and parvocellular subdivisions of the primate dorsal LGN remain largely segregated through several levels of cortical processing. It has been suggested that this segregation persists through to the highest stages of the visual cortex, and that the pronounced differences between the neuronal response properties in the parietal cortex and inferotemporal cortex may be attributed to differential contributions from magnocellular and parvocellular signals. We have examined this hypothesis directly by recording the responses of cortical neurons while selectively blocking responses in the magnocellular or parvocellular layers of the LGN. Responses were recorded from single units or multiunit clusters in the middle temporal visual area (MT), which is part of the pathway leading to parietal cortex and thought to receive primarily magnocellular inputs. Responses in the MT were consistently reduced when the magnocellular subdivision of the LGN was inactivated. The reduction was almost always pronounced and often complete. In contrast, parvocellular block rarely produced striking changes in MT responses and typically had very little effect. Nevertheless, unequivocal parvocellular contributions could be demonstrated for a minority of MT responses. At a few MT sites, responses were recorded while magnocellular and parvocellular blocks were made simultaneously. Responses were essentially eliminated for all these paired blocks. These results provide direct evidence for segregation of magnocellular and parvocellular contributions in the extrastriate visual cortex and support the suggestion that these signals remain largely segregated through the highest levels of cortical processing.