Abstract After intraperitoneal administration of high doses of methylene blue (MB) in cats, rabbits, dogs, guinea pigs, rats, and mice, the possible formation of ferrihemoglobin and Heinz bodies (HB) were studied. At no time and in no animal was ferrihemoglobin observed. Enzyme inductions with phenobarbital and SKF 525A served as a lead to the dye's decomposition mechanism and for its transformation into the HB-releasing agent. From this it emerged that phenobarbital could significantly shorten the interval before the 100% appearance of HB. SKF 525A prevented the appearance of HB. The inference drawn from the results of experiments on rabbits was that MB demethylation is the first step and that further metabolic processes follow; these were discussed in respect to their conversion in the manner known from aniline or the deamination following formation of radical intermediate stages.