Factors affecting the retrograde transneuronal transport of wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP), from spinal motoneurones to interneurones, have been studied in the cat. To this end, the location of transneuronally labelled interneurones was compared in animals which were awake or remained anaesthetized after WGA-HRP had been injected into the semitendinosus or the medial gastrocnemius nerve. In the anaesthetized animals motor axons of the injected nerves were stimulated selectively, to activate only Renshaw cells, or together with group I afferents, to activate also laminae V-VI interneurones with input from these muscles. The interneurones labelled in this study were distributed in different spinal cord regions than the interneurones labelled in preparations in which group I afferents of antagonist muscles were stimulated, as described in a previous study. The reported observations extend evidence of Harrison et al. that the retrograde transneuronal transport of WGA-HRP is facilitated by synaptic activity.