Competitive and noncompetitive antagonists were used to study the receptors which mediate the contraction elicited by 5-hydroxytryptamine (5-HT) and tryptamine (TRP) in the isolated rabbit aorta. The response to 5-HT was more susceptible to inhibition by competitive antagonists selective for 5-HT receptors, such as cyproheptadine, 5-methylgramine, 5-methoxygramine or 2-bromolysergic acid diethylamide, than was the response to TRP; the estimated apparent dissociation constants (KB value) for each antagonist was significantly lower when 5-HT rather than TRP was the agonist. If either of the two agonists and an antagonist were competing for the same single receptor, the KB value should be independent of the agonist. The 5-HT response was also more sensitive to the noncompetitive antagonist, dibenamine. A noncompetitive antagonist of alpha adrenergic receptors, benextramine tetrahydrochloride monohydrate (BHC), depressed the maximal TRP response 25 to 35% without affecting the 5-HT response. After blockade of alpha adrenergic receptors with BHC, KB values determined for each of the competitive antagonists using either 5-HT or TRP were no longer significantly different. Also after blockade by BHC, TRP and 5-HT responses were now equally sensitive to dibenamine. After adrenergic nerve terminals had been removed by stripping off the adventitia of the aorta, the response to TRP was still partially antagonized by BHC. It is concluded that in this preparation TRP directly activates both alpha adrenergic and 5-HT receptors and the 5-HT response is mediated by the 5-HT receptor with no involvement of alpha receptors.