A decrease of 30% was observed in the density of peripheral-type benzodiazepine binding sites in platelets of nine schizophrenics maintained on chronic neuroleptic treatment as compared to platelets of six untreated schizophrenics and 15 healthy volunteers. The equilibrium dissociation constant for binding by [3H]PK 11195, which is a ligand specific for peripheral benzodiazepine binding sites, was similar in all three groups. The relevance of the reduction in peripheral-type benzodiazepine binding sites to central or peripheral side-effects induced by long-term neuroleptic treatment merits further investigation.