Four methods of determining the concentration of mercury in human urine have been studied. A simple method suitable for general laboratory use is recommended and the requirements for accurate results are defined. The method employs mild oxidation with permanganate and HS 2 O 4 followed by dithizone extraction and measurement of absorbance at 485 nm and 620 nm. No mercury was detected in any of 74 urines from unexposed laboratory controls and hospital patients. A random urine sample seems adequate for the investigation of clinical or industrial mercury poisoning. Two individuals, free of symptoms, but subjected to moderate exposure, excreted 3·0-9·7 μg of mercury per 100 ml of urine. After the administration of an organic mercurial to two volunteers, urinary excretion was rapid and virtually complete within 48 hours.