Mice unable to synthesize dopamine (DA) in dopaminergic neurons were generated by gene-targeting techniques (Q.-Y. Zhou & R. D. Palmiter, 1995). These dopamine-deficient (DA-/-) mice required daily administration of 3,4-dihydroxyphenylalanine (L-DOPA) for survival beyond 2 to 3 weeks of age. This treatment stimulated mounting and aggressive behavior of adult DA-/- males toward both male and female mice. Both a nonspecific DA agonist (apomorphine) and a specific D1 agonist (SKF81297) stimulated aggression and mounting behavior; however, a D2 agonist (quinpirole) was less effective. Castration of male DA-/- mice demonstrated that these L-DOPA-stimulated behaviors depend on testosterone. In addition, replacement of testosterone to castrated males showed that the testosterone-responsive pathways of DA-/- males were more sensitive to testosterone than wild-type mice.