The metabolism of [U-(14)C]phenylmercury acetate was studied in the rat. After a single subcutaneous dose a small proportion is excreted unchanged in urine, and a larger amount in bile with some resorption from the gut. The greater part of the dose is broken down in the tissues to yield inorganic mercury which is excreted mainly in faeces, and conjugates of phenol and quinol are excreted in urine. In experiments in vitro phenylmercury is broken down by liver homogenates to release inorganic mercury and benzene; this reaction is effected by the soluble, but not the microsomal, fraction and does not require NADPH or NADH. No elemental mercury is formed under these conditions. It is probable that this reaction occurs in vivo and the benzene produced is rapidly converted into phenol and quinol by microsomal enzymes.