The concentrations for 50 percent inhibition of binding (IC50's) to specific in vitro serotonin binding sites (5-HT1 and 5-HT2) of rat cerebral cortex were determined for the trace amines 2-phenylethylamine, m- and p-tyramine, tryptamine, and (+)- and (-)- alpha-methyltryptamine. Tryptamine gave an IC50 of 66.7 +/- 4.8 nM (n = 7) at the 5-HT1 site and an IC50 of 3.85 +/- 0.16 microM (n = 7) for the 5-HT2 binding site. The IC50 values for all the other compounds were in the micromolar range and were different at the two binding sites except for p-tyramine (IC50, 5-HT1 = IC50, 5-HT2 = 17 microM. The trace amines may have different functional roles as evidenced by their different degrees of displacement of serotonin at 5-HT1 and 5-HT2 binding sites in the brain.