Previous studies have consistently reported that passive exposure to aggression is a risk of aggressive inclinations for a human witness. However, it is unclear whether a witness’ aggressiveness is semi-permanently socialized or temporarily primed. Furthermore, a neurochemical mechanism of passive exposure to aggression also remains unaddressed in clinical literature. The present research used a rat model to clarify the behavioral and neurochemical effects of passive exposure to aggression. First, rats were screened for their aggressiveness after they were acutely or chronically exposed to aggression or non-aggression. It was found that observer rats chronically exposed to aggression exhibited more aggression than those exposed to non-aggression and even those exposed to aggression only acutely. This behavioral difference was maintained over 16 days. Next, radioimmunoassay and autoradiography were used to test the levels of serum testosterone and corticosterone, as well as the densities of dopamine D₂ receptors and 5-HT_1B receptors, among observer rats chronically exposed to aggression or non-aggression. No differences in the hormonal levels were detected between the groups of exposure to aggression and non-aggression, whereas observer rats chronically exposed to aggression showed lower densities of dopamine D₂ receptors and higher densities of 5-HT_1B receptors, compared with controls. These suggest that chronic passive exposure to aggression inclined observer rats to be aggressive in the long run, which may be mediated by low densities of dopamine D₂ receptors and high densities of 5-HT _1B receptors.