A monkey made paretic by unilateral pyramidotomy will in time recover control over its affected limbs. This recovery is thought to be due in part to activity within the corticorubrospinal pathway, a system thought to be similar in function to the pyramidal tract (PT). In agreement, we have found that the corticorubral (CR) projection is derived principally from non-PT neurons, distributed topographically over the motor cortices in a manner similar to PT cells. Moreover, the red nucleus (RN) is known to be a site of remarkable, plastic changes in synaptic connectivity following disturbances of its afferent inputs or its motor targets. Anatomical studies in our laboratory have shown, however, that the bulk of the CR projection is not to the spinally projecting, magnocellular division of the RN as previously thought, but instead to the parvocellular division, which projects in turn to the olivo-cerebellar system. Thus, the normal motor functions of the CR system as well as the descending routes by which it might exert these functions in either the normal or the recovering animal are now unclear. To investigate this problem further, we will use single-unit recording techniques and the 2-deoxyglucose method with both the alert normal and the alert recovered animal to obtain data on the apparent functions of the corticorubrual system. The experiments proposed here will yield information in the following areas: (1) on the receptive fields and movement-related behavior of CR neurons in both the normal and the pyramidotomixed animal; (2) on the movement-related behavior of cells within the two divisions of the RN, divisions which appear to be parts of two distinct, motor control cir cuits; (3) on the possibility of an interneuronal linkage from the parvo- to the magnocellular division of the RN, a linkage that would re-establish the existence of a reasonably direct, corticorubrospinal route, and (4) on the major sites and kinds of altered unit activity within corticorubral, olivocerebellar, and rubrospinal pathways which might underlie the post-pyramidotomy recovery process. New data will thus be obtained on the normal functions of this major, extrapyramidal system, and on the kinds of recovery processes - studied here in an anatomically well-defined system - which might underlie the partial recovery in motor function seen after CNS trauma and stroke.