Phase-dependent reflex modulation was studied by recording the electromyographic (EMG) responses in soleus (SOL) and gastrocnemius medialis (GM) to a 20 ms train of 5 electrical pulses, applied to the sural or tibial nerve at the ankle, in 14 volunteers walking or running on a treadmill. Although both the spontaneous activity and the reflex responses were usually similar for both muscles, instances were identified in which separate control was evident. During walking (4 km/h), activity in SOL started earlier in the stance phase than GM activity. Correspondingly, the amplitude of the reflex responses was larger in SOL than in GM in early stance, both ipsi- and contralateral to the side of stimulation. In some cases, the same stimulus could elicit contralaterally a suppression of GM in synchrony with a facilitation of SOL. These crossed extensor reflexes had a low threshold (1.2 x T) and a latency ranging from 72 to 105 ms. During running (8 km/h or more), responses were seen selectively in GM instead, without concomitant responses in SOL. Such responses had a latency ranging from 82 to 158 ms and they appeared during the first extension phase, at the end of the swing phase. In addition, selective GM responses, with latencies above 200 ms, were seen near the transition from stance to swing during running. These instances of separate reflex control of SOL and GM were correlated with step cycle periods during which the motoneurones of either one of these muscles received more spontaneous activation than the other. Nevertheless, it is argued that premotoneuronal gating must also be involved since the increased amplitude of the crossed SOL responses (in early stance) and of GM responses (at end swing) was not strictly linked to an elevated amount of spontaneous activity during these parts of the step cycle as compared to other parts.