The influence of hind leg extensor muscle afferents on the timing of locomotor phase transitions was examined in adult, decerebrate rats, walking on a treadwheel. Walking occurred either spontaneously or was induced by stimulation of the mesencephalic locomotor region. Large diameter muscle afferents innervating the lateral or medial gastrocnemius were electrically stimulated during walking. A stimulus was delivered either at the onset of extensor muscle activity, or randomly during the step cycle. Stimulation with a train duration of 300 ms at the onset of extension increased the duration of the extensor bursts. The subsequent flexion phase was delayed. Stimulation with a shorter stimulus train (150 ms) early in extension had little effect on the extension phase duration. However when delivered at the end of extension the same stimulus significantly increased the duration of the extension phase and decreased the duration of the following flexion phase. Stimulating near the end of the flexion phase delayed onset and decreased duration of the subsequent extension phase. The effects of stimulating extensor afferents during the extension phase were weaker but qualitatively similar, to those in cats, suggesting similar mechanisms. The results of this study also show major differences in the integration of extensor muscle afferents between adult and neonatal rats.