The reactivity of the isolated guinea pig seminal vesicles to angiotensin and tyramine was investigated after chronic pretreatment of the animals with some potential microsomal enzyme inducers. After pretreatment with phenobarbital, 3-methylcholanthrene, SKF 525-A, glutethimide, and DDT, there was a marked increase in the reactivity of the isolated seminal vesicles to angiotensin and (or) tyramine. However, other inducers, such as meprobamate and chlorcyclizine, failed to alter the reactivity of the preparation. This effect of the inducers is possibly related to an increased protein synthesis, since the action of phenobarbital was completely abolished by actinomyein D. It is suggested that the microsomal enzyme inducers might stimulate the synthesis of the enzymes responsible for the formation of catecholamines in the seminal vesicles.