This study investigates the effects of 17mg and 13mg Delta THC on orientation and motor coordination and brain metabolic activity in regular users of marijuana. Subjects were tested twice, after smoking cigarettes with marijuana and after smoking cigarettes without marijuana. On both occasions they performed on a virtual reality maze task. They were afterwards scanned in Positron Emission Tomography (PET) with Fluoro-deoxy-glucose (FDG). 12 subjects were tested so far on both conditions.