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SUMMARY1. The aim of the study was to investigate whether inhibition of dorsal spinocerebellar tract (d.s.c.t.) cells evoked from group I afferents is mediated by the same interneurones which mediate the non-reciprocal inhibition of hind-limb motoneurones.2. The origin of inhibition of d.s.c.t. cells from group I afferents was compared in intact preparations, after lesions of the dorsal funiculi (when it could only be mediated by lower lumbar interneurones) and after lesions of the lateral and ventral funiculi (when it would be expected to be evoked by upper lumbar interneurones). In all three preparations extensors were the most common source of inhibition, as in motoneurones.3. Lower lumbar interneurones inhibiting d.s.c.t. cells were found to be co-excited by group I (I a and/or I b) and cutaneous and joint afferents, and by rubrospinal tract fibres, as are interneurones mediating inhibition of motoneurones. Co-excitation by group I and rubrospinal fibres was also found for upper lumbar interneurones.4. I.p.s.p.s were evoked in hind-limb motoneurones from within Clarke's column in cats with the dorsal funiculi cut between L4 and L5 segments; they were evoked at thresholds as low as 2 1sA, i.e. by stimuli with very local actions. The latencies of these i.p.s.p.s were short enough to allow them to be evoked monosynaptically via axonal branches of the same interneurones which projected to Clarke's column.5. Correspondingly, i.p.s.p.s were evoked in d.s.c.t. cells from within motor nuclei in L7 segments; they were evoked at similarly low thresholds and with similar latencies.6. In confirmation of previous reports i.p.s.p.s of I a origin evoked in d.s.c.t. cells were not found to be depressed by Renshaw cells, which excludes their mediation by interneurones responsible for I a reciprocal inhibition.7. The study leads to the conclusion that the inhibition of d.s.c.t. cells from group I afferents is, at least in part, collateral to the non-reciprocal inhibition of lumbar motoneurones.

3. Distribution of the caudally located interneurones in the lower lumbar segments was investigated after marking these interneurones with horseradish peroxidase retrogradely transported from Clarke's column. The horseradish peroxidase was injected along L3-L4 segments of Clarke's column in two cats with transacted dorsal funiculi. The marked cells were found in L5, L6, L7 and SI segments, with a highest density in L6 and L7. They were seen in laminae V, VI and VII. 4. A search was made for interneurones which could be antidromically invaded following stimuli applied in Clarke's column and were monosynaptically excited by group I afferents. Such interneurones were found at locations corresponding to laminae V-VI of Rexed. The latencies of antidromic and orthodromic responses were within ranges allowing them to mediate disynaptic inhibition of d.s.c.t. cells.