By use of a sensitive and specific enzymatic isotopic method for the determination of tyramine, the small quantities of this amine which are present endogenously in rat tissues, including brain, heart, kidney and salivary gland, have been quantitated. The levels of tyramine in brain were increased to a similar extent by injecting animals with a monoamine oxidase inhibitor, pargyline, and a dopamine beta-hydroxylase inhibitor, FLA-63; in contrast, pretreatment of animals with alpha-methyl-para-tyrosine, a tyrosine hydroxylase inhibitor, did not lead to an increase in tyramine levels in brain. Pretreatment of rats with 6-hydroxydopamine resulted in a marked diminution in the tyramine content of rat atria and salivary gland. Denervation of the salivary gland decreased the endogenous level of tyramine approximately 50% in denervated glands compared to undenervated glands. These results suggest that tyramine exists at least partly in sympathetic nerves in many tissues.