1. The level of the ascending projection of lumbosacral interneurones with a monosynaptic input from group I muscle afferents of the cat hindlimb has been investigated. The study was concerned particularly with the interneurones mediating group I non-reciprocal inhibition of motoneurones. 2. In chloralose-anaesthetized cats, single-unit microelectrode recordings were made, in the lateral funiculus at L6, from the ascending axons of lumbosacral interneurones. The axons studied were discharged by group I afferents primarily from extensor muscles of the knee and ankle. Some units were discharged in addition by cutaneous and/or joint afferents. 3. The extent of the ascending projection of the interneurones was assessed by antidromic activation of their axons in the lateral funiculus at different spinal levels. The great majority of axons tested (thirty-four out of thirty-six) were found to have an ascending projection to at least the L4 level and of these most (90%) did not project beyond the L4 or L3 segments of the cord. 4. The projection to at least L4, together with monosynaptic input from group I afferents and a pattern of peripheral input characteristic of interneurones in laminae V-VI of lumbosacral segments, identified thirty-four of the thirty-six units as interneurones mediating group I non-reciprocal inhibition. 5. These results suggest that the ascending group I inhibitory pathway, formed by these interneurones, is associated specifically with the group I relay of the dorsal spinocerebellar tract in Clarke's column, rather than being conterminous with group I afferents, which project throughout the rostral lumbar and lowest thoracic segments.