Clozapine exhibited 10-fold higher affinity than haloperidol for human 5-HT1A receptors expressed in Chinese Hamster Ovary cells (CHO-h5-HT1A) (K(i)s = 160 and 1910 nM respectively). Whereas haloperidol did not alter the basal binding of [S-35]GTP gamma S to CHO-h5HT(1A) membranes, clozapine stimulated it with an EC(50) of 2320 nM and an efficacy of 49% (compared to 5-HT). The stimulation was antagonized by the selective 5-HT1A receptor antagonist, WAY 100635 (1 nM).