The influence of the muscle length on the propagation velocity of the extraterritorial action potentials was studied. Single low--threshold motor units (MUs) from m. biceps brachii, both with superficially and deeply situated fibers were investigated. The velocity of excitation propagation decreases with the muscle elongation. This dependence is not so prominent for MUs having more deeply situated fibers. In case of superficial MUs the experimentally obtained values are in better agreement with the core conductor model (v = k.square root of r), than with the linear dependence of the propagation velocity (v) on the muscle fiber radius (r) (v = k.r) observed by Håkansson (1957) on frog muscles. The observed difference between superficial and deep MUs was discussed. At least two factors may be responsible for greater changes of the excitation propagation velocity along the fibers of the superficially situated MUs during the muscle elongation. One possible factor is the essential change in the external resistance due to the decrease of distance from the fibers to the skin surface. The greater change of the muscle length during a passive muscle stretching might be another reason.