Cytoxan (cyclophosphamide) given to rats intraperitoneally produced a severe cystitis within four hours with marked inflammatory edema and hemorrhagic ulcerations of the mucosa. An in vivo staining test with methylene blue showed deep staining of the urothelium as has been demonstrated with other types of urothelial injuries; uninjured urothelium does not stain. The cytoxan cystitis is probably not due to cytoxan itself, but to a breakdown product acrolein, an aldehyde appearing in the urine. Rat experiments demonstrated that acrolein instilled intravesically produced a cystitis similar to that found with cytoxan injected intraperitoneally. The cystitis due either to cytoxan or acrolein was prevented by simultaneous intravesical administration of an aldehyde inactivating agent, acetylcysteine (mucomyst).