Two mutations of the rat serotonin 5-HT2A receptor were made, expressed and examined for their ability to bind and be stimulated by certain tryptamines as well as their ability to bind antagonists. Mutation of Ser207 to an Ala (S207A) resulted in no substantial changes in binding of either 5-HT2A antagonists or agonists. In contrast, mutation of Ser239 to an Ala (S239A) resulted in significant changes in the 5-HT2A receptor with some but not all agonists and antagonists examined. Specifically, 5-HT had decreased affinity for the S239A mutated 5-HT2A receptor, showing over a 10-fold decrease in receptor-binding displacement, while still being capable of stimulating IP3 formation. However, the agonists tryptamine, 5-methoxytryptamine (5-MeOT), and N-1-isopropyl-5-methoxytryptamine; and the antagonists ketanserin, LY 86057, and LY 53857 were significantly less affected by a S239A mutation. These results suggest that while 5-HT might have a direct interaction with the Ser239 of the 5-HT2A receptor, tryptamine and 5-MeOT interact with this receptor in a different manner.