Clozapine and olanzapine behave as weak H3-receptor antagonists in vitro with Ki values around 1 and 50 microM, respectively. Despite these modest apparent affinities, both compounds given orally to mice, nearly doubled steady-state tele-methylhistamine levels in brain, with ED50 values as low as 1 and 3 mg/kg, respectively, an effect comparable to those of potent H3-receptor antagonists. This effect corresponded to an enhancement of histamine turnover rate from 45 to 73 ng/g/h as measured in the case of olanzapine using the pargyline test. Other antipsychotics displaying, such as clozapine and olanzapine, high 5-hydroxytryptamine (5-HT)2A receptor antagonist potency, i.e., risperidone, thioridazine, seroquel, and iloperidone, also enhanced markedly tele-methylhistamine levels. This effect was 1) additive with that of a pure H3-receptor antagonist, ciproxifan, 2) mimicked by a 5-HT2A receptor antagonist, ketanserin, 3) reversed by a 5-HT2A receptor agonist, DOI, 4) not shared by antipsychotics with low affinity for the 5-HT2A receptor, i.e., haloperidol, sulpiride, raclopride, or remoxipride that, on the contrary, tended to reduce tele-methylhistamine levels. We conclude that in contrast to "typical" antipsychotics, "atypical" antipsychotics stimulate histamine neuron activity via blockade of the 5-HT2A receptor in vivo. This effect does not appear to account for their reduced extrapyramidal side-effects but may underlie their pro-cognitive properties.