Summary After the administration of naphthalene to rats there is excreted in the urine a compound which can be decomposed by acid to yield naphthalene. The optimal acidity for the decomposition of this compound at room temperature was found to be pH 1.5-2.5, and under these conditions the amounts of naphthalene liberated did not increase after 8 hours. When the urines obtained after the administration of naphthalene to rats by stomach tube or by admixture in the diet were acidified under the optimal conditions described above, from 12.7 to 16.6% of the naphthalene administered was liberated. In similar experiments with phenanthrene and with anthracene the acidification of the urines obtained resulted in the liberation of from 3.9 to 7.3% of the phenanthrene and from 2.3 to 3.3% of the anthracene administered. No liberation of hydrocarbon was detected when the urines excreted by rats on diets containing acenaphthene, chrysene, 3,4-benz-pyrene, 1,2,5,6-dibenzanthracene or methylcholanthrene were acidified to pH 1.5-2.5. If the ingestion of these hydrocarbons by rats is followed by the excretion of acid-decomposable hydrocarbon precursors either such compounds are excreted in very small amounts or their decomposition occurs under conditions which differ from those which bring about the breakdown of the naphthalene precursor excreted after the administration of naphthalene.