Threshold depolarization is defined as the amplitude of the depolarization that will just bring a neurone to its firing threshold. Here, a novel technique is described for the measurement of threshold depolarization in resting human motoneurones. The tibial nerve was stimulated at an intensity that was just sufficient for the resulting la afferent volley to activate a single motor unit in the soleus muscle. The same motor unit was then recruited by voluntary activation, and the amplitude of the EPSP elicited by this stimulus was determined by a manner described earlier. The value found for the threshold depolarization was around 5 mV for all 6 motoneurones that were successfully tested. © 1991.