Experiments compared a series of phenethylamine hallucinogens with their phenylisopropylamine analogues for binding affinity and ability to stimulate serotonin 5-HT 2A receptor-mediated hydrolysis of phosphatidyl inositol in cells expressing cloned rat and human 5-HT 2A receptors. The (+/-)phenylisopropylamine analogues had significantly higher intrinsic activities for 5-HT 2A receptor-mediated hydrolysis of phosphatidyl inositol compared to their phenethylamine analogues. With respect to the effects of the stereochemistry of the phenylisopropylamines, those with the (R) absolute configuration at the alpha carbon had higher intrinsic activities for hydrolysis of phosphatidyl inositol in a cell line expressing the human 5-HT 2A receptor compared to those with the (S) absolute configuration. In virtual docking studies comparing the (R)- and (S)-phenylisopropylamines with their phenethylamine analogues, there were distinct differences in the orientations of key ligand binding domain residues that have been identified as important by previous mutagenesis studies. In conclusion, our data support the hypothesis that phenylisopropylamines have higher hallucinogenic potency than their phenethylamine analogues primarily because they have higher intrinsic activities at 5-HT 2A receptors. Although virtual ligand binding led to significant perturbations of certain key residues, our results emphasize the conclusion reached by others that overall three-dimensional structural microdomains within the receptor must be considered.