A procedure is described for the determination of submicrogram. quantities of mercury in urine. The method is based on the absorption of the 2537 A mercury resonance line. The urine specimens are extracted by an ammonium pyrrolidino dithiocarbamate/methyl/isobutyl ketone (APDC/MIBK) system and the MIBK extract is evaporated on a tantalum boat. The mercury is detected by an atomic absorption spectrophotometer. Measurement of the absorption peak height is used to determine the quantity of mercury present. A 50 ml urine sample is required, which allows for many replicate runs.