Corrections for specific gravity, osmolality, and creatinine were applied to identical urinary mercury data in order to investigate the reproducibility of the corrected mercury concentration in a person's urinary spot samples throughout a 24-hour period; and the extent to which it is possible to predict a person's 24-hour mercury excretion from the corrected mercury concentration in the corresponding spot samples. The data indicate that on the average these corrections improve reproducibility by a factor of approximately 2. In individual cases, the uncorrected and corrected values of mercury concentration can differ by as much as a factor of 6. Similar effects were observed in the correlation of the mercury concentration of a spot sample and the corresponding 24-hour excretion of mercury. For this set of data, the corrections for specific gravity and osmolality turned out to be almost identical. The correction for creatinine was more effective than the other corrections by a small but statistically significant amount.