Pentachlorophenol (PCP) has been shown to be methylated to O-methyl-PCP, pentachloroanisole (PCA), in various biological systems. The disposition and metabolism of PCA were studied in female mice to which the compound was administered at a dose of 20 mg (14C)PCA/kg. Elimination of (14C)PCA equivalents from mouse tissues was rapid, with half-lives ranging from 5 to 10 hr in all tissues examined except liver. Excretion of 14C was primarily via the urine, in which a PCP conjugate, free PCP, and an oxidation product, tetrachlorohydroquinone, were demonstrated. Free PCP and its conjugate were also present in feces. There was no evidence for the presence of parent PCA in either urine or feces. Thus the half-lives of elimination and metabolite patterns resulting from treatment of mice with PCA approximated those seen following PCP administration to rodents. These data suggest that PCA must be demethylated prior to excretion.