The behavioral profile seen in humans following administration of marijuana has long been a source of intense interest, because the unique syndrome produced does not resemble that seen after any other psychoactive substance. Subjectively, the cannabinoid syndrome in humans includes sensory enhancement, errors in the judgment of time and space, dissociation of ideas, delusions, impulsivity, and hallucinations (Pertwee 1988). For example, individuals often report that while under the influence of cannabis, auditory, gustatory, and visual stimuli often seem more intense and pleasurable; whereas, time often seems to proceed more slowly. Memory loss, especially short-term or working memory, and attentional deficits are also hallmarks of cannabinoid intoxication (Chait and Pierri 1992;Schwartz 1993). The central nervous system mechanisms and sites of action of these cognitive effects are still poorly understood. NO . 6 Regional changes in cerebral blood flow and metabolism are thought to reflect changes in regional neuronal activity (Sokoloff 1981). Because specific cognitive or drug-induced challenges are known to induce regionally specific changes in neuronal activity regional cerebral blood flow (rCBF), and glucose utilization (Sokoloff, 1981), various functional metabolic imaging procedures have been employed in an attempt to understand better marijuana's neuroanatomical sites of action. Mathew and colleagues (Mathew et al. 1989(Mathew et al. , 1992) measured changes in rCBF following acute drug administration. An increase in CBF was seen in experienced versus inexperienced subjects with greater frontal and right hemisphere blood flow seen 30 min after marijuana smoking. These blood flow effects were not related to changes in general circulation, respiration, or plasma tetrahydrocannabinol (THC) concentrations (Mathew and Wilson 1993). In contrast, using intravenous ⌬ 9 THC, the principal psychoactive agent in marijuana, Volkow et al. (1991) reported an increase in glucose utilization (rCMRglu) only in the cerebellum when data were corrected for changes in global metabolism. More recently, in a population of THC abusers, Volkow et al. (1996) demonstrated THC-induced increases in rCMRglu in the orbitofrontal and prefrontal cortex and basal ganglia that correlated with the subjective sense of intoxication. Baseline cerebellar rCMRglu rates were also lower in abusers than control subjects. In contrast to the above studies on human subjects, Margulies and Hammer (1991) reported biphasic, dose-dependent effects of ⌬ 9 THC in most limbic (but not diencephalic/brainstem) regions, with low doses causing an increase and high doses a decrease in rCMRglu. We (Bloom et al. 1997) have also demonstrated heterogenous decreases in rCBF after acute ⌬ 9 THC administration in the rat at doses comparable to the high doses of Margulies and Hammer (1991).Despite these gains in our understanding of the neuroanatomical localization of ⌬ 9 THC's effect upon brain activity, the recent discovery of an endogenous cannabinoid receptor in ...