AbstractImpulsivity is a personality trait characterized by acting suddenly in an unplanned manner in order to satisfy a desire without consideration for the consequences of such behavior. There are several psychiatric disorders that include the term impulsivity as a criterion and, therefore, it has been suggested that impulsivity may be an important phenotype that may link a number of different behavioral disorders, including substance abuse. This study's aims were to determine if a significant association could be detected between the (AAT)n triplet repeat polymorphism as well as 5 single nucleotide polymorphisms (SNPs) in or near theCNR1receptor gene and impulsivity in Southwest California ‘Mission’ Indians (SWC). Impulsivity was assessed using a scale derived from the Maudsley personality inventory, and blood samples were collected for DNA analyses from 251 individuals recruited from local Indian reservations. The estimated heritability (h²) for the impulsivity phenotype was 0.20 + 0.12 (p< .004). Impulsivity was significantly associated with the 6-repeat allele of the triplet repeat polymorphism (AATn/A6;p< .0001), as well as four SNPs in or near theCNR1receptor gene: rs1535255 (p= .001), rs2023239 (p= .004), rs1049353 (p< .001) and rs806368 (p< .0006). These studies provide data to suggest that theCNR1receptor gene may be significantly associated with impulsivity in SWC Indians.