Tremor-producing activity and concentration in brain were determined in mice for 4 harmala and 5 iboga alkaloids. All compounds were active, however, harmalol only after intracerebral injection. Kinetics of evasion from brain were first-order functions with most drugs, but revealed 2 compartments for harmalol and 3 for ibogaline. Tremor-producing activity was much more influenced by chemical structure than by lipid solubility. This points to specific receptors for indole compounds in tremorigenic brain structures.