Because dopamine D2 receptors are known to be elevated in schizophrenic brain striata, this study examined whether a similar dopamine receptor elevation occurred in other diseases including neuroleptic-treated Alzheimer's and Huntington's diseases. The average D1 density in postmortem striata from Alzheimer's patients was 17.6 +/- 0.1 pmol/g, similar to an age-matched control density of 16.6 +/- 0.4 pmol/g. The average D1 density in schizophrenia patients was 19.0 +/- 0.6 pmol/g, similar to the age-matched control density of 17.9 +/- 0.6 pmol/g. In Parkinson's disease patients, however, the D1 receptor density was elevated, with values of 22.8 +/- 1.2 pmol/g (in patients not receiving L-DOPA) and 19.6 +/- 1.5 pmol/g (in patients receiving L-DOPA) compared to the age-matched control density of 16.0 +/- 0.4 pmol/g. The D2 receptors in Alzheimer's striata averaged 13.4 +/- 0.6 pmol/g (in patients who had not received neuroleptics), almost identical to the control density of 12.7 +/- 0.3 pmol/g. The average D2 density in neuroleptic-treated Alzheimer's striata was 16.7 +/- 0.7 pmol/g, an elevation of 31%, the individual values of which had a normal distribution. In Parkinson's disease patients, the D2 densities were elevated in tissues from patients not receiving L-DOPA (19.9 +/- 1.5 pmol/g in putamen and 14.8 +/- 1.2 pmol/g in striatum) compared to the age-matched control values of 13.0 +/- 0.4 pmol/g and 12.6 +/- 0.3 pmol/g, respectively. In Huntington's disease patients, the D2 density averaged 7.5 +/- 0.4 pmol/g in patients who had not received neuroleptics, but was 10.3 +/- 0.6 pmol/g in those who had. Although all of the D1 and D2 densities in each of the above diseases and subgroups revealed a normal distribution pattern, the D2 densities in schizophrenia displayed a bimodal distribution pattern, with 48 striata having a mode at 14 pmol/g, and the other 44 striata having a mode at 26 pmol/g. Thus, compared to the neuroleptic-induced and unimodal elevations in D2 of 31% in Alzheimer's disease and 37% in Huntington's disease, the schizophrenic striata with a mode of 26 pmol/g (105% above control) appear to contain more D2 receptors than can be accounted for by the neuroleptic administration alone.