The abilities of some naturally occurring beta-carbolines (BCs), dihydro-BCs and tetrahydro-BCs to inhibit the specific binding of 3H-tryptamine (TA), 3H-serotonin (5-HT) and 3H-ketanserine to rat brain membranes and to induce tremor in mice were studied. These compounds, particularly DHBCs and BCs, showed higher affinity for TA binding sites than to 5-HT1 or 5-HT2 binding sites inhibiting the former at nanomolar and the two latter ones at micromolar or high micromolar concentrations. The Ki values for norharmane, harmaline and harmine (17, 18 and 74 nM, respectively) for TA sites indicate the highest affinity so far described for natural beta-carbolines to any receptor sites and thus may indicate their major site of action. among the BC derivatives studied, the before mentioned harmala alkaloids were the most potent inducers of tremor in mice, although the orders of the tremorogenic potency and the binding to TA site did not correlate. It is suggested that especially the tremorigenic effect of BC derivatives is partly based on the binding to specific tryptamine receptors.