The 5-HT2 receptor in isolated rabbit thoracic aorta was characterized by examining the relationships between structure and activity of nine tryptamine analogs. All assays were conducted after blockade of the alpha adrenergic receptor and inactivation of the neuronal uptake-1 system and monoamine oxidase. Seven of the analogs tested were agonists. 6-Hydroxytryptamine and 7-hydroxytryptamine showed little or no agonist activity in this preparation. The pA2 of spiperone was independent of the agonist assayed and defined the receptor activated by each agonist as the 5-HT2 receptor. The dissociation constant (KA) and relative intrinsic efficacy were determined for each agonist. The KA and relative intrinsic efficacy values for 5-hydroxytryptamine were 0.25 microM and 1, respectively. The KA and relative intrinsic efficacy values for 5-methoxytryptamine were 0.14 microM and 0.86, respectively, and were not significantly different from those for 5-hydroxytryptamine. The other five analogs were partial agonists. N-Methyl-5-hydroxytryptamine and bufotenine had relative intrinsic efficacies of about 0.3 and KA values not statistically different from the KA value for 5-hydroxytryptamine. Tryptamine, 5-methyltryptamine and alpha-methyl-tryptamine had KA values of about 1 microM and relative intrinsic efficacies of 0.6, 0.6 and 0.4, respectively. These results revealed the differential effects of structural changes on drug affinity and intrinsic efficacy. This information will be applicable in the design of selective agonists or antagonists for the classification of less well defined 5-hydroxytryptamine receptors.