Biological monitoring for exposures permits estimation of organ doses or body burdens from exposures through all relevant portals of entry. Biological monitoring data may be used to estimate environmental concentrations when the latter cannot be measured directly. Biological indices are usually surrogates for the concentration of a chemical or its metabolites or its effect at the true receptors. Mercury concentration in urine has-been most-coinmoialy-recommended as a biological exposure index of mercury. For data based on urine analysis, variation in urine volume is the most significant. The urinary concentration related to excretion of the solute provides some correction for fluctuation of urine output. Sampling time must be carefully observed because distribution and elimination of a chemical are kinetic events. This study has evaluated mercury concentration in spot urine compared to the results of 24 hour collected urine by the adjustment methods (specif ic gravity, creatinine) and sampling time. The subjects were 43 workers who had been exposed to the metallic mercury. The results were as follows: 1. The correlation coefficients between mercury concentration in 24 hour urine and that in spot urine were 0.639-0.715 and were not different by adjustment methods. 2. In the high exposure group who were over lOOug/1 of urinary mercury, the correlation coefficients between mercury concentration in 24 hour urine and that in spot urine were 0. 687-0.824 and were not different by adjustment methods. 3. Mercury concentration in spot urine were very variable by sampling time or exposure time. The correlation coefficients between mercury concentration in 24 hour urine and that in spot urine were most highest as 0.85-0.91 at first voiding urine in the morning, and were 0. 77-0.86 at urine collected within four hours before end of shift. In the biological monitoring to exposure of mercury, sampling of spot urine were most proper at first voiding urine in the morning, and then at urine collected within four hours before end of shift. But the adjustment methods of specific gravity and creatinine were no difference of the results.