Behavioral, biochemical and recent electrophysiological data have increasingly implicated the involvement of dopamine in the central actions of cannabinoid compounds. However, the site and mechanism by which cannabinoids stimulate dopamine systems has been somewhat controversial. Central opioid systems have also been suggested to play a role in some cannabinoid-induced behaviors as evidenced by their attenuation in the presence of the opioid antagonist naloxone. However, recent studies using the cannabinoid receptor-selective antagonist SR141716A suggest that the central actions of psychoactive cannabinoids are mediated principally through activation of CB1 receptors. Using single cell electrophysiological recordings in the rat we assessed the effects of both SR141716A and naloxone on delta9-tetrahydrocannabinol (THC)-induced activation of ventral tegmental dopamine neurons. While dopamine cell firing was dose-dependently increased following cumulative dosing with delta9-THC it was partially or completely inhibited following pretreatment with 0.5 and 2 mg/kg SR141716A, respectively. However, 1 and 10 mg/kg naloxone failed to alter the response to delta9-THC. These data provide the first evidence that delta9-THC-induced changes in mesolimbic dopamine neuronal activity are mediated by the CB1 cannabinoid receptor, but a causal link for the involvement of opioid systems could not be established.