1. 3-Ethyl-5-hydroxy-4,5-dimethyl-Δ3-pyrrolin-2-one (HPL, ‘mauve factor’) was determined quantitatively in the urine of schizophrenic, general medical and porphyric subjects by a sensitive gas/liquid-chromatographic method using a nitrogen-specific detector.

2. A comparative evaluation with previously used methods for HPL was made and some problems of specificity are discussed.

3. The concentration of HPL in early morning and spot samples of urine from 146 subjects with schizophrenia was not greater than that in 42 general medical patients, contrary to previous reports.

4. Of the three patients with acute intermittent porphyria, two excreted HPL. One subject, studied over a 2 year period, did so intermittently in a manner unrelated to her attacks of porphyria.

5. It is concluded that the urine content of HPL is unlikely to be causally related to schizophrenia or to the clinical manifestations of acute intermittent porphyria.

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