Evidence supporting the validity of the 'two dopamine receptor' hypothesis is presented. The availability of the 'first generation' of selective agonists and antagonists of the D-1 and the D-2 dopamine receptors provides pharmacological support for the hypothesis. The demonstration that stimulation of the D-2 receptor either inhibits or has no effect upon adenylate cyclase activity while stimulation of the D-1 receptor enhances cAMP formation provides biochemical support for the hypothesis. Finally, binding assays demonstrating two affinity states for the D-1 and the D-2 receptors are briefly discussed.