Dopamine beta-hydroxylase (DBH; EC 1.14.17.1) catalyzes the production of the neurotransmitter and hormone norepinephrine in the third step of the catecholamine biosynthesis pathway. Transgenic mice were generated with multiple copies of a human DBH minigene construct containing the full-length cDNA connected downstream of the 4-kilobase upstream promoter region to achieve overexpression of DBH. Human DBH mRNA and immunoreactivity were detected tissue-specifically in the brain and adrenal gland of these transgenic mice. The transgene products were correctly processed to a glycosylated mature polypeptide with a molecular mass of 72 kDa and existed in the secretory vesicles as both soluble and membrane-bound forms. We detected a marked increase in DBH activity in various catecholamine-containing tissues of the mice that occurred as a consequence of expression of the catalytically active human DBH enzyme. However, in these transgenics the steady-state levels of norepinephrine and epinephrine were normally maintained without the acceleration of the catecholamine turnover rate, suggesting that there are some regulatory mechanisms to preserve a constant rate of norepinephrine synthesis in spite of the increased amount of DBH protein. These transgenic mice with the minigene construct provide one approach to study the mechanisms underlying biogenesis of the DBH polypeptide and regulation of norepinephrine synthesis.