The loss of midbrain dopamine in Parkinson's disease is accompanied by a matching loss in the dopamine transporter and a rise in the D1 and D2 receptor densities. This is found in the brain putamen and caudate tissues from unmedicated patients, and may account for the good early clinical response to L-dopa. Long-term L-dopa treatment reverts the receptor densities toward normal levels. Positron emission tomography (PET) data and in vitro data generally concur. In schizophrenia the density of the dopamine transporter as well as that of the D1 dopamine receptor is normal. The D2 receptor density, however, is consistently elevated in postmortem brain putamen and caudate nucleus, even in tissues from neuroleptic-free or drug-naive patients. Three sets of PET and single photon emission computed tomography (SPECT) data support the postmortem findings. Early evidence indicating abnormal D2 structure as well as a reduced link between D1 and D2 warrant a detailed study of the genes for these two receptors in schizophrenia.