AbstractUrinary selenium excretion was studied in 21 mercury vapour (Hg°)‐exposed workers involved in the demolition of a chloralkali plant. The subjects had no known previous occupational exposure to mercury. Their mean pre‐exposure urinary mercury concentration, determined on average 1.2 days prior to the exposure, was 0.8 nmol mmol−1creatinine (range 0.3–1.9). Their last mean urinary mercury concentration, determined on average after 51.4 days (range 19–103) of exposure, was 4.8 nmol mmol−1creatinine (range 1.2–10.0). The exposure ceased on average 4.1 days after the last determined urinary mercury concentration. The corresponding concentrations of urinary selenium decreased from an average of 39.1 nmol mmol−1creatinine (range 13.9–89.5) to 29.0 nmol mmol−1creatinine (range 10.1–52.9) (P= 0.002). This finding may indicate that even a low to moderate work‐related exposure to Hg° may reduce the urinary selenium concentration in humans in a manner th