In the rabbit 2:3:5:6-tetrachloronitrobenzene is metabolized by reduction of the nitro group, hydroxylation and mercapturic acid formation (Bray, Hybs, James & Thorpe, 1953). The last reaction is of particular interest since it is accompanied by loss of a nitro group directly attached to the benzene ring. Two further examples of this type of reaction were encountered when pentachloronitrobenzene and 2:3:4:6-tetrachloronitrobenzene were given to rabbits (Betts, James & Thorpe, 1953). A more detailed study of the metabolism of these two compounds is now reported. The stability of the nitro group in pentachloro-, the tetrachloroand three trichloro-nitrobenzenes has been examined in relation to formation of mercapturic acids from the highly chlorinated nitrobenzenes.