1. [4- 14 C]δ-Aminolaevulinate (ALA) was administered by the oral route to three patients with symptomatic porphyria cutanea tarda as a tracer dose and the incorporation of radioactivity into urinary 8-, 7-, 6-, 5- and 4-carboxyl porphyrins studied as a function of time. In one of these patients the study was repeated after partial depletion of stored liver porphyrins by low-dose chloroquine therapy. 2. Maximal radioactive incorporation into the urinary porphyrins was generally inversely related to the degree of carboxylation. In all instances maximal specific radioactivity of coproporphyrin far exceeded that of uroporphyrin, but was achieved at the same time interval. 3. Chloroquine therapy resulted in a porphyrinuria with a composition substantially similar to that before treatment. After partial depletion of the liver porphyrin stores maximum specific radioactivity of all urinary porphyrins increased as compared with before treatment. 4. Non-porphyric livers, obtained at post-mortem, were analysed for porphyrin content and composition; appreciable performed pools of 8-, 7-, 6-, 5-, 4- and 2- carboxyl porphyrins were found. 5. Results of the radioactive studies are interpreted as consistent with dilution in preformed pools of stored porphyrins of decreasing size. It is suggested that the predominance of 8- and 7-carboxyl porphyrins in the livers and urines of patients with porphyria cutanea tarda is due to rate-limitation of uroporphyrinogen decarboxylation in those liver cells which store free porphyrins.