Feeding hexachlorobenzene to rats is well known to produce porphyria, an effect which does not decline readily after feeding the chemical has been stopped. The difficulties in elimination of hexachlorobenzene is taken as the reason why the porphyria lasts so long after feeding has been discontinued. Individual rats on this diet show great variation in the magnitude of the porphyrin excretion, the interval to maximum porphyrin elimination and the decline of porphyrin elimination which may not reach base line and may take months to reach a steady state level. On the other hand some animals do not respond to feeding hexachlorobenzene with urinary porphyrin excretion, which is an unusual observation.