Distributions of serum dopamine-β-hydroxylase(DBH) activity and thermostability in a randomly selected population of Korean adults were studied to investigate the genetic factor which could exert an influence on the serum DBH activity and thermostability. The results were followings: 1. The mean serum DBH activity in a randomly selected population of adults was 18.3 ± 4.5 umol/min/l(mean ± SD;n=327) which showed no age or sex variation. 2. The frequency distribution showed no isolated subgroup with very low serum DBH activity. 3. When the ratio of enzyme activity after heating 55˚C for 20 minutes to that before heating (heated-to-control or H/C ratio). was used as a measure of thermostability, the mean serum DBH H/C ratio in a randomly selecfld population, of adults was 0.90 ± 0.17(mean ± SD; n=327) which showed no age or sex variation. 4. Serum DBH H/C ratio showed unimodal, homogeneous distribution. 5. There was significant negative correlation between serum DBH activity and H/C ratio(r=-0.39. p<0.01). 6. Subjects with H/C ratio less than 0.1 had significantly higher basal enzyme activity (22.2 ± 4.5) (mean ± SD;n=33) umo1/min/1 than those with H/C ratio more than 1.09 (15.5 ± 3.3) (mean ± SD;n=32) umo1/min/1. This study shows that the distribution patterns of serum DBH activity and thermostability of Korean population are considerably different from those of Caucasian and it might be a line of evidence for the different inheritance pattern of plasma DBH enzyme between these racial groups.