Isolated strips of the human tracheal muscle had slow fluctuations of membrane potential (slow wave), essentially similar to those of guinea-pig trachea. The average amplitude and frequency of slow wave were 7.7 mV and 19.9/min (n = 9), respectively. There was clear correlation between the electrical activity and mechanical response during the excitation caused by tetraethylammonium (TEA), carbachol, and noradrenaline and also during the inhibition by isoprenaline. The slow wave seems to be related to the basal muscle tone, which is probably maintained by endogenous leukotrienes in human and by prostaglandins in guinea-pig.