The actions of diverse antipsychotics at cloned h5-HT1B and h5-HT1D receptors were examined employing [H-3]-GR125,743 and [S-35]-GTP gammaS for determination of affinities and efficacies, respectively. Compared with hD(2) receptors, haloperidol, chlorpromazine and olanzapine showed markedly (> 100-fold) lower affinity for h5-HT1D and h5-HT1B receptors at which they expressed inverse agonist properties. Clozapine, risperidone and ocaperidone likewise behaved as inverse agonists at h5-HT1B and h5-HT1D receptors but their affinities were only similar to 10-fold lower than at hD2 receptors. Moreover, ziprasidone, S16924 and ORG5222 interacted at h5-HT1B and h5-HT1D receptors with affinities similar to hD(2) sites. While S16924 and ORG5222 were inverse agonists at h5-HT1B anti h5-HT1D sites, ziprasidone was an inverse agonist at h5-HT1D receptors yet a partial agonist tit h5-HT1B receptors. These actions of antipsychotics were abolished by the selective, neutral antagonist, S18127. In conclusion, with the exception cf ziprasidone, till antipsychotics were inverse agonists at h5-HT1B; and h5-HT1D receptors, although they differed markedly in their potency tit these sites as compared to hD(2) receptors. (C) 2001 American College of Neuropsychopharmacology. Published by Elsevier Science Inc.