In experienced marijuana smokers, marijuana smoking was accompanied by a significant bilateral increase in cerebral blood flow (CBF) especially in the frontal regions and cerebral blood velocity. The post-marijuana CBF increase could not be explained on the basis on changes in general circulation or respiration. Similarly, the CBF increase was unrelated to plasma levels of tetrahydrocannabinol and extracranial circulation. Behavioral changes showed significant correlations with CBF. CBF and brain function are closely coupled and therefore it seemed highly likely that CBF changes after marijuana were closely related to its effect on mood and behavior.