Biological monitoring of mercury in whole blood (B-Hg) or urine (U-Hg) can be used to assess exposure to mercury vapor if the kinetics and other sources of variation are taken into account. Its rapid rise postexposure makes B-Hg a good indicator of recent exposure peaks, while U-Hg (corrected for urinary flow rate) reflects average long-term exposure. However, high intraindividual variation sometimes requires the average of several U-Hg determinations. In the general population, methylmercury from fish and mercury from dental amalgam influence B-Hg and U-Hg, respectively, and must be considered if other exposures are being monitored. The quantitative relations between mercury in biological fluids and critical organs are poorly understood. Monitoring U-Hg is useful for assessing the risk of adverse effects and the need for preventive measures. At average U-Hg levels of about 50 micrograms.g creatinine-1 (28 nmol.mmol creatinine-1) the prevalence of symptoms and slight objective changes in the central nervous system and the excretion of certain urinary proteins are increased.