1. A new method for the measurement of uroporphyrinogen decarboxylase (EC 188.8.131.52) in rat liver homogenates, with 5-carboxyl porphyrinogen as substrate, is described.
2. The administration of a diet containing 0·3% (w/w) hexachlorobenzene produces porphyria in female Wistar rats after a delay of at least 4 weeks. The development of porphyria is accompanied by a progressive fall in hepatic uroporphyrinogen decarboxylase activity to 18% of control values after 11 weeks. The features of hexachlorobenzene porphyria are consequences of this enzyme defect.
3. Feeding with hexachlorobenzene did not lead to the accumulation of iron in the liver. It is suggested that hexachlorobenzene or a metabolite acts directly to decrease the activity of the enzyme.