The complete cortical extent of the origin of the corticospinal tract (CST) and the quantitative contribution to the CST of different cytoarchitectonic areas were established, using the technique of retrograde labeling with horseradish peroxidase (HRP) in the monkey. HRP was applied to the transected stump of the cut CST at the level of the first segment of the cervical spinal cord (Cl). Labeled cells were plotted on the projection drawings of every 10th frontal sections and reconstructed on the cortical surface of the hemisphere. The CST cortex occupied the upper two thirds of Brodmann's areas 6, 4, 3, and 1-2 combined. It also covered completely areas 5, 31, and 32, as well as, a part of areas 7 (the parietal operculum) and insula. These areas included not only the entire cortical fields corresponding to the limbs and trank representations of the precentral motor (MI), supplementary motor (MII), primary somatosensory (SI), and the second somatic sensory (SII) areas, but expanded more widely than the delimitation for the CST cortex by the earlier studies. The contralateral and ipsilateral hemispheres contained 74,710 and 3,130 CST cells respectively. Area 4 on the contralateral hemisphere contained about a half (50.8%) of the total CST cells, and 79.0% of CST cells of both hemispheres was located in the cortex anterior to the central sulcus (MI and MII); area 6 (11.9%), area 4 (51.6%), area 3 a (5.9%), area 32 (9.6%). Remaining 18.1% of CST cells occurred in the postcentral areas (SI); area 3 b (2.6%), area 1-2 (4.4%), area 5 (8.8%), area 31 (2.3%). Area 7 and insula (SII) contained only 3% of CST cells. About an half of CST cells resided in the sulcal cortex. Earlier findings on the CST cells were also confirmed, i.e., the CST cells are located exclusively in the deeper portion of the cortical layer 5, are distributed in a cluster, and are all pyramidal shape of various sizes. Soma diameters of labeled CST cells were determined, and the size distribution in each area revealed that the large-sized CST cells were located only in areas 4 and 3 a. Areas 6, 3 b, 32, 31, and insula contained only the small-sized CST cells, and other areas contained the small and medium types. To determine the origin of the total fibers contained in the pyramidal tract (PT), HRP was also applied to the cut stump of the PT at the level of the upper-most medullar pyramid. The results are discussed in comparison with the findings on the CST system.