1. The decarboxylations of uroporphyrinogens, hepta-, hexa- and penta-carboxyporphyrinogens I and III by porphyrinogen carboxy-lyase (EC 4.1.1.37) in rat liver supernatant have been compared as functions of substrate concentrations. Although Km and Vmax. (for total porphyrinogens formed) were estimated, prophyrinogens and CO2 produced at 1 microM were considered to be a better indication of real relative rates, owing to substrate/product inhibitions. Uroporphyrinogen III was the best substrate by the criteria of Km/Vmax. and decarboxylation at 1 microM and was converted into coproporphyrinogen more quickly than its series-I isomer. 2. The difference between uroporphyrinogens I and III as substrates was confirmed by using a mixture of [14C8]uroporphyrinogens, the discrimination occurring principally in the first decarboxylation. 3. Porphyrins, especially oxidation products of the substrates, inhibited the enzyme. Heptacarboxyporphyrin III was the most effective inhibitor of both uroporphyrinogen III and heptacarboxyporphyrinogen III conversion into coproporphyrinogen. 4. Rapid analysis of the livers from rats made porphyric with hexachlorobenzene demonstrated that substantial quantities of the tetrapyrroles were present in vivo as the porphyrinogens (21-42%). 5. Enzymic decarboxylation of uroporphyrinogen III in 2H2O-containing buffer gave [2H4]coproporphyrinogen. 6. Rats treated with cycloheximide for 10h showed no decrease in uroporphyrinogen decarboxylase activity/mg of protein, suggesting a relatively slow turnover of the enzyme.