1. Few data are available that describe the evoked activity of spinal cord sensory tract neurons as a function of behavioral state. Accordingly, experiments were performed in which ascending volleys were recorded extracellularly within the spinoreticular (SRT), spinothalamic (STT), and spinomesencephalic (SMT) tracts located in the ventrolateral reticular formation in response to low-intensity electrical stimulation of the contralateral sciatic nerve in the chronic unanesthetized, behaving cat during naturally occurring episodes of wakefulness, quite sleep, and active sleep. 2. During episodes of wakefulness and quite sleep sciatic nerve stimulation produced a low-amplitude and long-duration orthodromic field potential that did not differ in amplitude or waveform. However, during the corresponding episode of active sleep, the sciatic nerve-induced orthodromic field potential was markedly suppressed or abolished. 3. The effects of sustained microiontophoretic applications of inhibitory amino acid agonists, glycine, or gamma-aminobutyric acid during wakefulness or quite sleep markedly suppressed the antidromic field potential recorded from nearby VII motoneurons but did not alter the sciatic nerve-evoked orthodromic field potential, indicating that the sciatic response was recorded from ascending axons of passage emanating from lumbar spinal neurons. We suggest that lumbar neurons comprising the SRT, STT, and SMT tracts are subjected to a descending suppressor drive that is activated specifically during the behavioral state of active sleep.