Studies of the effects of inheritance on human catecholamine biosynthetic and metabolic enzymes are reviewed. Genetic polymorphisms have been shown to control the level of the biosynthetic enzyme dopamine beta-hydroxylase in serum and the level of the metabolic enzyme catechol-O-methyltransferase in a variety of tissues. Platelet monoamine oxidase, another catecholamine metabolic enzyme, is also under genetic control. These observations help to explain individual differences in response to treatment with catechol drugs and may explain some variations in the function of human catecholamine neurotransmitters. The biochemical genetic approach used to study the enzymes of catecholamine biosynthesis and metabolism could be used to investigate other aspects of adrenergic function and could serve as a model for studies of the effects of inheritance on other human neurotransmitter systems.