Drug-elicited head-twitch behavior is a useful model for studying hallucinogen activity at 5-HT 2A receptors in the mouse. Chemically diverse compounds active in this assay yield biphasic dose-effect curves, but there is no compelling explanation for the "descending" portion of these functions. A set of experiments was designed to test the hypothesis that the induction of head-twitch behavior is mediated by agonist actions at 5-HT 2A receptors, whereas the inhibition of head-twitch behavior observed at higher doses results from competing agonist activity at 5-HT 2C receptors. The effects of the phenethylamine hallucinogen R (