Intracisternal injection of 5,7-dihydroxytryptamine (5,7-DHT) following treatment with desmethylimipramine induced development of behavioral supersensitivity to the intraperitoneally administered serotonin precursor 5-hydroxytryptophan (5-HTP) in the mouse. This behavioral syndrome, characterized by tremor and muscle twitches (myoclonus), showed a clear dose-response relationship with 5,7-DHT as well as with 5-HTP. Mice lesioned with a low dose of 5,7-DHT (20 micrograms) or a placebo were treated repeatedly with a protein synthesis inhibitor, sycloheximide (45 mg/kg, s.c., every 12 h for up to 10 days). This treatment resulted in a reversible decrease of cerebral protein synthesis varying between 70 and 20% with time between treatments. The myoclonic response to 5-HTP in animals pretreated with 5,7-DHT and by cycloheximide showed a decrease in intensity within 24 h when evaluated quantitatively by an electronic activity monitor, the results of which were confirmed by direct observation. Cycloheximide also exerted a similar, though smaller, effect following full development of sensitivity to 5-HTP over 10 days. These effects may de mediated by inhibition of rapidly turning over serotonin receptor proteins, although their interpretation is somewhat obscured by possible toxic effects of cycloheximide.