Mercury (Hg) concentrations in individual spot urine samples collected over consecutive I'd periods were compared with Hg concentrations measured in combined 24-h urine samples from 69 practicing dental professionals with low exposure to Hg vapor (Hg°) in order to validate the use of spot urine samples as an indicator of Hg exposure. The level of Hg° in air as an exposure measure was also evaluated by comparing air concentrations of Hg° in dental offices with both spot and 24-h urine Hg° levels. The results showed: (1) There was little diurnal variation (∼9%) in urinary Hg values; (2) a strong correlation (R2=.85) exists between the Hg concentration in the first morning void and that in a complete 24-h urine sample; (3) adjustment of urinary Hg levels for creatinine concentrations did not improve this correlation; (4) there was no added value in the speciation of total urinary Hg into the inorganic Hg fraction; and (5) concentrations of Hg° in air did not significantly correlate with measures of Hg in urine at this low Hg° exposure level. We conclude from this study that first morning void urine samples may be used to derive reasonably valid