1. The contractile potencies of some sympathomimetic amines and cholinergic drugs were studied in the isolated, stripped vas deferens of the guinea-pig.2. alpha-Methylnoradrenaline, adrenaline and metaraminol produced the same relative maximal responses as noradrenaline, which served as reference drug; on the other hand, octopamine, dopamine and alpha-methyldopamine were much less active. Short term pretreatment of the animals with reserpine did not affect the maximal relative responses.3. Tyramine, alpha-methyl-m-tyramine and hordenine methiodide had little effect on the untreated preparation. In the presence of just threshold concentrations of noradrenaline, the relative maximal responses to tyramine and hordenine methiodide were markedly increased.4. The relative maximal response to carbachol was the same as that to noradrenaline, while that to DMPP was smaller. In contrast to carbachol, the action of DMPP was potentiated by pretreatment of the preparation with noradrenaline.