Normal range of mercury contents in blood and its relationship with urinary mercury excretion were studies with 68 healthy male adults living in Seoul city, who had no obvious evidence of either occupational exposure to mercury or therapeutic use of mercurial agents. Mercury analysis was made by means of dithizone colorimetric method with coefficient of variation of 10.9% in an average ranging from 5.1% to 18.0%. 1. Mercury contents in normal human blood were both normally and log-normally distributed, and better fitted to the latter. 2. Geometric mean and standard deviation of the mercury contents were 24.0%(log(-1) 1.38)+/-1.66 microgram/100 ml (log(-1) 0.22 microgram/100 ml) ranging from 7.2 to 79.7 microgram/100 ml with 95% confidence interval. 3. Mercury contents in normal human blood differed from person to person (p<0.01), and the variability of the measurements was negligible (p>0.05). 4. Mercury in the blood was contained much higher in erythrocytes than I!in plasma (p<0.01), showing the geometric means of 21.0+/-1.25 microgram/100 ml in red blood cells and 14.3+/-1.62 microgram/100 ml in plasma, respectively. 5. Mercury contents in normal human blood had a relationship of power function with mercury excretion in urine corrected with a gram of creatinine excretion per liter of urine (p<0.10).