The major aim of this study was to define the topography of descending motor cortical influences on the dorsal column nuclei of the rat. A total of 1442 single neurons were recorded throughout the rat cuneate (Cu) or external cuneate (ECu) nuclei and classified according to their: 1) location within the nuclei, 2) receptive fields, and 3) response to standardized deep layer microstimulation in the forelimb area of the motor (MI) cortex. Excitatory neuronal responses to this MI cortical stimulation ranged in latency from 2-28 msec, with 80% of responses in the 4-12 msec range. Overall, the rostral extremity of the Cu (0.5-1.0 mm rostral to the obex), and the ECu contained the highest percentage of recorded neurons responding to the MI cortical stimulation (61%). By contrast, only 11% responded in the middle subregion of the Cu (+0.4 to -0.9 mm relative to the obex), and 28% responded in the caudal-most subregion (1.0-2.0 mm caudal to obex). A similar paradigm was used to investigate the topography of MI corticofugal modulation of afferent transmission through the Cu and ECu. The MI cortical stimulation was used as a conditioning (C) stimulus in a condition-test (C-T) paradigm in which the test (T) stimulus was standardized mechanical vibration of digit No. 4 of the contralateral forepaw. A total of 30 cells from different subregions were analyzed by measuring the averaged evoked unit responses to the T-stimuli, and plotting them as a function of C-T intervals from 0-50 msec.(ABSTRACT TRUNCATED AT 250 WORDS)