The latency of consecutive H-reflex responses of single human triceps surae motoneurones varies up to 2,500 μs. A large part of this variation was shown to occur at the synaptic transmission. A moderate increase in stimulus strength from the threshold value shortened the mean latency and reduced the latency variation, presumably as a result of spatial summation of excitatory inputs. Further increase to maximum strength lengthened the mean latency, increased the variation, and resulted in a dropping out of some responses which was not produced by collision by antidromic impulses. These effects are believed to be due to an active inhibition. Changes of the latency were also obtained by Jendrassik's manoeuvre and facilitatory and inhibitory conditioning stimuli.