1. Parkinson's disease is associated with profound decreases in striatal dopamine concentrations. 2. There are significant increases in the densities of striatal D1 and D2 receptors as part of the compensatory mechanism. 3. Levodopa therapy induces a down-regulation of both D1 and D2 receptors to normal densities. 4. Positron emission tomography detects striatal changes in early Parkinson's disease or toxin-induced parkinsonism.