Impulsivity, a highly prevalent symptom in multiple psychiatric disorders, is a partially heritable trait influenced by specific biological mechanisms. In particular, dopamine is proposed to play a role in impulsive behaviors and recent studies have implicated functional polymorphisms of dopamine-related genes in impulsive behaviors across different clinical and behavioral classifications. However, most have not isolated the impulsivity construct per se as a biologically based and measurable endophenotype. The present study was therefore undertaken in a sample of healthy adults to investigate the influence of two candidate dopaminergic gene polymorphisms (DRD4 and DAT) on the endophenotype of impulsivity, which we operationalized as behavioral inhibition during the Stop-signal task. We recruited an ethnically diverse sample of 119 healthy adults to complete a self-report questionnaire of impulsivity and to perform a Stop-signal task. We report significant differences in inhibitory control between individuals with at least one 7-repeat allele of the DRD4 polymorphism, as well as an interaction between DRD4 and DAT genotypes, on inhibitory control. Results of the present study support the influence of dopaminergic variation on impulsive-related measures, as well as the advantage of using measures which are likely more sensitive to the effects of such genetic variation. © 2007 Wiley-Liss, Inc.