Previous work in cats has shown that cells of origin of the corticospinal tract give rise to collateral branches to the dorsal column nuclei (DCN). The present experiments were performed in monkeys (Macaca fascicularis) in which 2% fast blue and 2% diamidino yellow were delivered to infiltrate the dorsolateral funiculus at levels between C2 and C6 and the cuneate nucleus on the same side. Retrograde labelling in the cortex allows simultaneous visualization of three classes of neurons: corticospinal tract (CST) neurons, corticocuneate tract (CCT) neurons, and double-labelled neurons. The morphological features and distribution of CST and CCT neurons are similar to those previously reported from investigations based mainly upon the retrograde transport of horseradish peroxidase (HRP). CST neurons occur in layer V in the pre- and postcentral gyri, except for the lateral part (face representation), in the supplementary motor and sensory cortex, and in SII. CCT neurons are present in layer V largely in the postcentral gyrus and in SII. Double-labelled neurons are present wherever CST and CCT neurons are found. Reconstruction and quantitative data from the pericentral cortex show that up to 60% of CCT neurons are double-labelled and are found predominantly in areas 1 and 2, and that their perikarya are in the size range of the larger CCT neurons. Comparison of these results with those obtained previously in cats by using HRP and tritiated, enzymatically inactive HRP (3H-apo-HRP, Rustioni and Hayes: Exp. Brain Res. 43:237-245, 1981) suggests that CST neurons with branching axons to the DCN are considerably more numerous in monkeys than in cats. To determine whether this difference is caused by the different tracers used in the two species. 2% fast blue and 2% diamidino yellow were delivered in cats to infiltrate the dorsolateral funiculus at C2-C3 and the cuneate nucleus on the same side. The results in these cats are remarkably similar to those obtained in the previous study, which used HRP and 3H-apo-HRP: double-labelled neurons occur predominantly in area 3a and constitute 14-16% of the CCT neurons in the pericruciate area. The results bear upon mechanisms of descending control and tuning of performances that characterize the dorsal column-medial lemniscal system, e.g., discrimination of discrete spatiotemporal cues. The species differences may be related to the higher degree of tactile resolution and synchronous control of sensory inflow at the DCN and spinal cord in monkeys relative to cats.