The mesocortical dopamine system is thought to play an important role in the etiology of the stress response. Dopamine (DA) has been shown to accumulate in the rat frontal cortex in response to a wide variety of stressors. Diazepam, an anxiolytic benzodiazepine, can reverse the effects of stress on cortical DA. We investigated the effects of acute and chronic diazepam administration on immobilization stress-induced changes of the DA system in the frontal cortex of the rat. In the first study, 2.5 mg/kg diazepam was administered 20 min prior to 40 min of immobilization stress. Acute diazepam significantly reduced basal levels of extracellular DA and antagonized the stress-induced increase in cortical DA when compared to untreated stressed rats. Acute diazepam did not significantly effect extracellular DOPAC. In the second study, an experimental group of rats was given approximately 2 mg/kg/day diazepam in their drinking water for 3 weeks. This treatment significantly reduced anxiety as assessed by a staircase test for anxiety. Chronic diazepam had no effect on basal levels of cortical DA. However, chronic diazepam treatment also attenuated stress-induced increases in extracellular DA when compared to untreated stressed control rats. Chronic diazepam did not affect stress-induced changes in DOPAC but it did antagonize the effects of stress on HVA. Thus, acute and chronic diazepam treatment can antagonize stress-induced activation of the mesocortical DA system. It is proposed that this effect is produced through an enhancement of GABAergic neurotransmission by diazepam. The role of the dopaminergic system during stress, anxiety, and schizophrenia is discussed.