The organization and collateralization of corticostriate neurones in the motor and sensory cortex of the rat brain was investigated using double retrograde labelling techniques. The results demonstrated that the motor and sensory corticostriate projections are strongly bilateral with an ipsilateral predominance. The labelled corticostriate cells were medium-sized pyramidal cells localized predominantly in lamina Va, with fewer cells in lamina Vb and only the occasional cell in laminae III and VI. A significant proportion (13%) of corticostriate neurones in the sensory-motor cortex of the rat were double-labelled, indicating that they provide collateral projections to the striatum on both sides of the brain. The highest percentage of double-labelled corticostriate neurones was found in the primary motor cortex (14%) with lower percentages in the primary sensory cortex (10%) and in the forelimb and hindlimb sensory-motor overlap areas (6%).