In this study we have investigated the effects of low threshold afferent stimulation on the centrally generated locomotor rhythm in an isolated spinal cord-hindlimb preparation from newborn (0-3 days old) rats. Locomotor activity was induced by bath application of N-methyl-D-aspartate (NMDA). Stimulation of low threshold afferents (below 2 times threshold (x T) for the incoming volley) in L2 and L3, but not L4 and L5 dorsal roots resets the ongoing rhythm. This is due to an abrupt excitation of the L2 and L3 ventral root bursts and an inhibition of the L5 ventral root burst on the ipsilateral side followed by reciprocal effects on the contralateral side. Similar effects were seen after stimulation of the quadriceps nerve with weak intensity (below 1.8 x T). We conclude that low threshold muscle afferents, possible group I afferents, have access to central pattern generators early in postnatal life and suggest that these findings may give an important opening in ontogenetic studies of how neural networks are assembled.