Although the basis of schizophrenia is not known, evidence indicates a possible overactivity of dopamine pathways. In order to detect any new dopamine receptor-like sites which may be altered in schizophrenia, the present study used a new radioligand, a [3H]benzo[g]quinoline. The receptors were labelled by this ligand in the presence of other drugs to block the known dopamine D1, D2, D3, or D5 receptors (no D4-selective ligands are available to block D4). Using this method, we found that schizophrenia brain striata had elevated levels of a D2-like site not detected in control human postmortem brains or in Alzheimer's, Huntington's, or Parkinson's disease brains. The ligand acted as an agonist at this D2-like site, because binding was abolished by guanine nucleotide. The binding of the ligand to the D4 receptor, however, was not sensitive to guanine nucleotide. The site differed from D2 itself, because S- and R-sulpiride were equally potent at the D2-like site. The D2-like sites were present in rat and mouse brain but were absent in brain slices from transgenic mice where D2 had been knocked out. The abundance of the receptor was not related to premortem use of antipsychotic drugs. Future research should examine the biochemical differences between the D2 dopamine receptor and these D2-like sites in schizophrenia.