The external and internal intercostal nerves of a single intercostal space were stimulated in anaesthetized paralysed cats with dorsal roots cut in the corresponding spinal cord segment. 2. Extracellular recording in the ventral horn revealed single units which fired short high frequency bursts of spikes at short latency to stimulation of either or both of the two nerves at stimulus strengths appropriate to the activation of alpha motor axons. These units were deduced to be Renshaw cells. 3. Small (0.1-0.2 mV) hyperpolarizing potentials of duration up to 50 msec were recorded intracellularly in both inspiratory and expiratory motoneurones of the same segment. Latencies and thresholds were appropriate for disynaptic i.p.s.p.s evoked by collaterals of alpha motor axons. 4. The changes in probability of firing following the stimuli were examined for inspiratory alpha motoneurones by constructing post-stimulus histograms of efferent discharges recorded from filaments of the external intercostal nerve of the segment stimulated and from other segments. 5. A period of reduced probability of firing of up to 24 msec duration, corresponding in all respects to disynaptic inhibition from alpha motor axon collaterals, was seen in the segment stimulated and up to three segments distant, though declining in intensity with distance. Either nerve could evoke such inhibition although that evoked from the internal intercostal nerve was stronger, as were the intensities of the Renshaw cell discharges. 6. We conclude that recurrent inhibition, via Renshaw cells which have axons up to 30 mm in length, is present for intercostal motoneurones. Arguments are adduced to show that although the effects from stimulating any one segmental nerve may be relatively weak, the over-all effect resulting from the widely spread projections of the Renshaw cells concerned is an inhibition comparable intensity with that seen in many hind limb motor nuclei.
