Dopamine-beta-hydroxylase (DBH), an enxyme catalyzing the final step in the synthesis of norepinephrine, is released with norepinephrine on stimulation of the sympathetic nervous system. In this study an enzymatic colorimetric method was used to assay serum DBH activity in 196 individuals of whom 169 were either the parents or children in 29 families. We examined the association of DBH with age and with hypertension and the faamilial pattern of distribution of serum DBH activity. Serum DBH activity was highest in te groups 10 to 14, 40 to 49, and over 60 years. There was no significant difference in serum DBH activity between normal subjects and individuals with hypertension in any age group. tfurthermore, there was no significant correlation of mean blood pressure with serum DBH activity with or without correction of DBH for age differences; Highly significant correlations of serum DBH activity were found in sibling-sibling pairs and in mean parent-child pairs. No significant correlations were found for father-mother pairs. These observations suggest that the contribution of heredity is more important that shared environment in determining the familial pattern of distribution of serum DBH activity. Serum DBH activity did not have a bimodal distribution in the population. Noevidence was found for a maternal influence or for sex linkage in the transmission of this trait. Our data, derived from this study restricted to families with only two generations, are not adequate to specify with confidence the mode of inheritance of DBH activity in the general population.