Abstract The metabolism of acetylcysteine was studied and compared with that of l-cysteine. Incubation of acetycysteine with rat liver homogenate resulted in the rapid formation of cysteine. In the rat, 2 hr after the oral administration of acetylcysteine- 35S, neither acetylcysteine- 35S nor its disulfide could be found in the liver, but cystine 35S and cysteine- 35S were tentatively identified. An appreciable quantity of acetylcysteine- 35S was found in lung. Inorganic sulfate was the major urinary excretion product; however, evidence was obtained for the presence of small amounts of acetylcysteine- 35S and taurine- 35S in the urine of rats given acetylcysteine- 35S orally. Oral administration of cysteine or acetylcysteine to rats or dogs produced little change in blood or tissue sulfhydryl levels, and only a small increase in urinary sulfhydryl compounds.