A method is described for the study of guinea-pig vas deferens in vivo response to phenylephrine and norepinephrine. Tissue sensitivity to both agonists did not differ in vivo but the maximum response to phenylephrine was smaller. Responsiveness to norepinephrine was depressed in vitro after in vivo experiments. No such effect was observed for phenylephrine. This method allowed the comparison of the responsiveness of the guinea-pig vas deferens to adrenoceptor agonists in vivo and in vitro.