1. Single-unit recordings have been made from 606 neurones in the arm region of area 4 in five conscious monkeys. Their activity during a stereotyped motor task and their responses to passive natural stimulation of the limb have been investigated.2. 88% of area 4 neurones responsive to natural stimulation received their afferent input from a restricted region of the contralateral arm.3. The activity and afferent input to cell groups have been determined by comparing the properties of neurones located within 500 mum of each other and recorded in one and the same micro-electrode penetration. 115 such cell groups containing 344 neurones were investigated.4. 75/115 cell groups (65%) contained neurones with input from the same arm zone (shoulder, elbow, wrist or hand) and with a similar pattern of task-related activity. Cell groups containing neurones with identical afferent inputs never showed contrasting behaviour during movement.5. 40/115 cell groups (35%) contained neurones receiving inputs from more than one arm zone. Twenty-five cell groups (22%) had two contiguous zones (e.g. wrist and hand) represented and ten groups had input from two discontinuous zones (e.g. elbow and hand). These differences in input within a cell group were usually reflected in contrasting behaviour of its constituent neurones during movement.6. Pyramidal tract neurones (PTNs) lying immediately adjacent to one another usually received similar inputs and exhibited matching behaviour. PTNs lying further apart in the same penetration often showed different activity and responded to different stimuli.7. The topographic distribution of afferent input to area 4 revealed multiple representation of input from a single zone combined with considerable intermingling of input from all four zones. Neurones with shoulder and elbow inputs surrounded those with wrist inputs which in turn lay scattered around a central zone. This central zone only contained neurones with hand inputs, although neurones with hand inputs were found outside this central zone.8. The significance of this complex organization is discussed in terms of motor cortex input and output.