Two cannabinoid receptors are reviewed with regard to their primary structure, ligand-binding properties, and signal transduction systems. Both receptors have been cloned; therefore, the expression of their genes and the functional domains within the proteins can be examined. Binding of tritiated agonists has localized these receptors to the central nervous and immune systems. The CBI receptor is predominantly expressed in brain tissues and is found in both glial elements and neurons; subcellular localization to axons and terminals is evident. This receptor is found in motor, limbic, associative, cognitive, sensory, and autonomic brain structures. CBI receptors modulate the activities of calcium and potassium channels. The CB2 receptor is predominantly expressed in the immune system and is found in spleen, tonsils, thymus, mast cells, and blood cells. Although receptors appear to be involved in cannabimimetic-induced modulation of immune cell function, the receptor subtype that is principally involved in specific effects is difficult to determine because both receptors are often coexpressed in the same cells. Cannabimimetic-induced effects on mast cells and B cells appear, however, to be mediated by CB2 receptors.