Data on the ability of Delta 9-tetrahydrocannabinol (THC) to modify reward processes in experimental animals are inconsistent. This study examined the effects of Delta 9-THC on brain reward function using the rate-frequency curve shift paradigm of intracranial self-stimulation (ICSS) and the conditioned place preference (CPP) paradigm. In ICSS tests, rats were implanted with electrodes into the medial forebrain bundle. After brain stimulation reward thresholds stabilized, rats received intraperitoneal injections of Delta 9-THC (0, 0.5, 1 and 2 mg/kg) or the CB1 receptor antagonist SR141716A (0, 0.02 mg/kg) and Delta 9-THC (0, 2 mg/kg). The two highest doses of Delta 9-THC significantly increased the threshold ICSS frequency. SR141716A reversed the action of Delta 9-THC (2 mg/kg), without affecting reward thresholds by itself. In the CPP test, mice received intraperitoneal injections of Delta 9-THC (0, 1 or 3 mg/kg). Delta 9-THC showed neither statistically significant preference nor aversion in either of the doses tested. These findings indicate that Delta 9-THC, in contrast to other drugs of abuse, does not facilitate ICSS or support CPP under the present experimental conditions, but rather has a dose-dependent inhibitory influence on ICSS.