A myoclonic syndrome consisting of tremor, myoclonus, and seizures was produced following the systemic administration of 5-hydroxytryptophan to adult rats previously given intracisternal injections of 5,7-dihydroxytryptamine and systemic desmethylimipramine, but not in their controls. This behavioral response was blocked by pretreatment with the putative serotonin receptor blocking agents methysergide, lysergic acid diethylamide, and bromolysergic and diethylamide, as well as centrally effective doses of the aromatic amino acid decarboxylase inhibitor Ro4-4602. Blockers of receptors of other neurotransmitters had little effect. This neurologic response in the adult rat may be relevant to some forms of clinical myoclonus and may be useful in testing potential agonists and antagonists of serotonin receptors in the mammalian central nervous system.