Thymoxamine hydrochloride administered by mouth to rats at 25 or 100 mg kg-1 was excreted in the urine as the deacetyl and N-demethyl-deacetyl metabolites. These were completely sulpho- and glucuronoconjugated at 25 mg kg-1 but only partially so at the higher dose. Thymoxamine deacetylation in vitro is catalysed by plasma and hepatic cytosol esterases and the deacetyl metabolite undergoes N-demethylation catalysed by the cytochrome P 450 hepatic microsome mixed function monooxygenase system. Because of the rapidity of the deacetylation it is concluded that thymoxamine is a prodrug leading in vivo to the active deacetyl thymoxamine.