Anandamide, an endogenous ligand for brain cannabinoid CB(1) receptors, produces many behavioral effects similar to those of Delta(9)-tetrahydrocannabinol (THC), the main psychoactive ingredient in marijuana. Reinforcing effects of THC have been demonstrated in experimental animals, but there is only indirect evidence that endogenous cannabinoids such as anandamide participate in brain reward processes. We now show that anandamide serves as an effective reinforcer of drug-taking behavior when self-administered intravenously by squirrel monkeys. We also show that methanandamide, a synthetic long-lasting anandamide analog, similarly serves as a reinforcer of drug-taking behavior. Finally, we show that the reinforcing effects of both anandamide and methanandamide are blocked by pretreatment with the cannabinoid CB(1) receptor antagonist rimonabant (SR141716). These findings strongly suggest that release of endogenous cannabinoids is involved in brain reward processes and that activation of cannabinoid CB(1) receptors by anandamide could be part of the signaling of natural rewarding events.