(4-Phenoxyphenylsulfonyl)methylthiirane (compound 1) is a highly selective and potent inhibitor of gelatinases that shows considerable promise in animal models for cancer and stroke. The metabolism of compound 1 was investigated in mice, following intraperitoneal administration at 100 mg/kg. Eight metabolites were identified in plasma and urine. The primary routes of metabolism of 1 were hydroxylation at the para-position of the terminal phenyl ring, hydroxylation at the alpha-methylene to the sulfonyl, which lead to the generation of a sulfinic acid, and cysteine conjugation of the thiirane ring. The cysteine adducts arose through addition of glutathione to the thiirane ring. The molecule is extensively metabolized and