The interactions of the indolealkylamine N,N-dimethyltryptamine (DMT) with 5hydroxytryptamine 1A (5-HTIA) and 5-HT 2 receptors in rat brain were analyzed using radioligand binding techniques and biochemical functional assays. The affinity of DMT for 5-HT 1A sites labeled by [3H]-8. hydroxy-2-(di-n-propylamino)tetralin ([3H)-8-0H-DPAT) was decreased in the presence of 10. 4 M GTP, suggesting agonist activity of DMT at this receptor. Adenylate cyclase studies in rat hippocampi showed that DMT inhibited forskolin·stimulated cyclase activity, a 5-HT 1A agonist effect. DMT displayed full agonist activity with an EelO of 4 x 10- 6 M in the cyclase assay. In contrast to the agonist actions of DMT at S-HTIA receptors, DMT appeared to have antagonistic properties al S-HT2 receptors. The ability of DMT to compete for [3H)-ketanserin-labeled 5-HT 2 receptors was not affected by the presence of 10-' M GTP, suggesting antagonist activity of DMT at 5-HT 2 receptors. In addition, DMT antagonized 5-HT 2 -receptor-mediated phosphatidylinositol (PI) turnover in rat cortex al concentrations above 10- 7 M. with 70% of the 5-HT-induced PI response inhibited at 10-' M DMT. Micromolar concentrations of DMT produced a slight PI stimulation that was not blocked by the 5-H1'2 antagonist ketanserin. These studies suggest that DMT has opposing actions on 5-HT receptor subtypes, displaying agonist activity at 5-HT 1I1 receptors and antagonist activity at 5-HT z receptors.