1. Receptive-field properties of antidromically identified efferent neurons within the cutaneous forelimb representation of primary somatosensory cortex (S-1) were examined in fully awake rabbits. Efferent neurons studied included callosal neurons (CC neurons, n = 52), ipsilateral corticocortical neurons (C-IC neurons, n = 48) that project to or beyond the second somatosensory cortical area (S-2), and corticofugal neurons of layer 5 (CF-5 neurons, n = 97) and layer 6 (CF-6 neurons, n = 59) that project to and/or beyond the thalamus. 2. An additional class of neurons was studied that was not activated antidromically from any stimulus site, but which responded synaptically to electrical stimulation of the ventrobasal (VB) thalamus with a burst of three or more spikes at frequencies of 600 to greater than 900 Hz. Most of these neurons also responded synaptically to stimulation of S-2 and the corpus callosum. The action potentials of these neurons were much shorter (mean = 0.45 ms) than those of efferent neurons (mean = 0.95 ms). Such properties have been associated with interneurons found throughout the central nervous system, and these neurons are thereby referred to as suspected interneurons (SINs). 3. CF-5 neurons differed from CC, C-IC, and CF-6 neurons in their spontaneous firing rates, axonal properties, and receptive-field properties. Whereas CF-5 neurons had a mean spontaneous firing rate of 5.5 spikes/s, CC, C-IC, and CF-6 neurons had mean values of less than 1/s. Axonal conduction velocities of CF-5 neurons were much higher (mean = 12.92 m/s) than either CC (mean = 2.15 m/s), C-IC (mean = 1.31 m/s), or CF-6 (mean = 2.53 m/s) neurons. A decrease in antidromic latency (the "supernormal" period) that was dependent on prior impulse activity was seen in the great majority of CC, C-IC, and CF-6 neurons but was either minimal or absent in CF-5 neurons of comparable conduction velocity. A higher proportion of CF-5 neurons (98%) responded to peripheral sensory stimulation than did either CC (75%), C-IC (71%), or CF-6 (51%) neurons. CF-6 and C-IC neurons that did not respond to sensory stimulation had significantly lower axonal conduction velocities and spontaneous firing rates than those that responded to such stimulation. 4. Cutaneous receptive fields were seen in most neurons that could be driven by peripheral stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)