A comparison is presented of the latencies of corticofugal effects from the contralateral somatosensory cortex (SI) onto the cat's dorsal column nuclei (d.c.n.) under pentobarbitone anaesthesia. The latencies for transmission in the ascending pathway from d.c.n. to SI after stimulation within the gracile and cuneate nuclei were found to be 3.3 ms for the former and 2.8 ms for the latter. The time courses of inhibition of a medial lemniscal mass response following cortical conditioning and evoked by stimulation of peripheral nerves were measured. All latencies were corrected to exclude the different times taken for stimuli to reach the nuclei from the two limbs. The optimal condition-test interval was 12 ms with a duration of 14.3 ms for the superficial radial nerve (s.r.n.) and 45 ms and 30 ms respectively for the medial plantar nerve (m.p.n.). In each case cortical conditioning inhibited the wave by about 50%. The effect of cortical conditioning upon spontaneously firing d.c.n. single units was investigated. For cuneate cells the mean latency was 6.8 ms and the mean duration 36.8 ms. For gracile cells the latency of onset of inhibition was 17.2 ms and its duration 129 ms. In 75% of cells mixed effects were seen with facilitation preceding inhibition. The latencies of 'corticofugal reflex' action on the gracile and cuneate nuclei after stimulation of the s.r.n. and m.p.n. were determined. The gracile response had a latency approximately 4 times that for the cuneate response. The temporal asymmetry of these corticofugal effects suggests that the pathway is not purely a simple feed-back loop, but may be concerned in other physiological contexts, some of which are discussed.