The effects of the primary psychoactive constituent of marijuana, delta 9-tetrahydrocannabinol, are mediated by cannabinoid receptors, CB1 and CB2. The CB1 receptors display a unique central nervous system (CNS) distribution and are present in mammalian brain at higher levels than most other known G-protein-coupled receptors. The highest levels occur in several areas involved in motor control and hippocampus. Cannabinoid effects on CNS activities, including movement, memory, nociception, endocrine regulation, thermoregulation, sensory perception, cognitive functions, and mood, correlate with the regional distribution of cannabinoid receptors and their activation of specific G-protein-mediated signal transduction systems in various brain regions.