The twitch-response enhancing potencies of (the agonists) substance P, bethanechol, 4-aminopyridine and of potassium chloride were compared in the field-stimulated guinea-pig vas deferens. The agonists were tested after the twitching activity of the isolated organ had been reduced to an equal extent by (the antagonists) tetrodotoxin, procaine, magnesium chloride, and by decreased stimulation frequency. Further experiments were performed in the functionally denervated organ. The potencies differed with the type of antagonism of the twitch-response. This pointed at different modes of action and/or sites of mechanism of the response-enhancing compounds. The pharmacological nature of agonists and antagonists as well as the results in the denervated organ indirectly suggested a predominantly neuronal point of mechanism of the agonists. This was most obvious for substance P.