We sought to identify behavioral and biochemical differences between Dark Agouti and Fischer 344 inbred rat strains to assess whether they could serve as a model of genetically determined differences in sensitivity to drugs of abuse. We compared the strains for the following traits: morphine-induced locomotor activity and sensitization; circadian variation in plasma levels of corticosterone, a hormone reported to affect sensitivity to drugs of abuse; and several biochemical parameters in the ventral tegmental area and nucleus accumbens, brain regions implicated in the locomotor activating and reinforcing actions of drugs of abuse. Fischer 344 rats exhibited greater initial locomotor responses to morphine but, unlike Dark Agouti rats, did not develop sensitization to a second morphine exposure. Fischer 344 rats displayed a marked rise in basal plasma corticosterone levels in the late light phase and early dark phase, whereas Dark Agouti rats showed no significant circadian variation in corticosterone levels. Relative to drug-naive Fischer 344 rats, drug-naive Dark Agouti rats showed higher levels of tyrosine hydroxylase and glial fibrillary acidic protein, and lower levels of neurofilament proteins, in the ventral tegmental area. In contrast, no strain differences were found in levels of tyrosine hydroxylase, specific G protein subunits or protein kinase A in the nucleus accumbens. Together, these results demonstrate that Dark Agouti rats and Fischer 344 rats exhibit differences in specific behavioral, endocrine and biochemical parameters related to sensitivity to drugs of abuse.