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Porphyrinogens were determined in very fresh, diluted morning urine in two types of hepatic porphyria, using a simple and very quick spectrophotometric method. It was shown that acidification by itself is insufficient for the complete conversion of colourless porphyrinogens into porphyrins in photometric determination of porphyrins in diluted urine. A correlation was shown between percentual content of porphyrinogens and the sum of pentacarboxyporphyrins and coprophyrin. A difference was found between the excretion of porphyrinogens in morning fresh urine in porphyria cutanea tarda (average porphyrinogen fraction 22.5%, SD 10.3% of total porphyrins) and in acute intermittent porphyria (average porphyrinogen fraction 77.1%, SD 9.3% of total porphyrins). The method is at the same time suitable for the detection of urobilinoids in urine. Oxidation of the urine in acute intermittent porphyria is recommended before absorption on talc and subsequent thin-layer chromatography, because porphyrinogens are not adsorbed on talc.