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Desmodium pulchellum has been reported to be a traditional remedy of schistosomiasis. Our previous pharmacological studies demonstrated a decreased locomotion, an increased blood pressure, and an increased respiration n rats treated with aqueous extracts from various parts of the herb, especially the root bark. This prompted us to isolate the active principles responsible for such activities. In this experiment, the root bark was extracted by maceration using a series of solvents. Dried methanolic extract was found to possess the above mentioned effects. At a dose of 30 mg/kg (p.o) induced stereotyped behaviors (paw padding, tail-flick, hind-limb extension) in rats. Higher doses caused convulsion and death within an hour. These similar behaviors induced by tryptophan, were potentiated by the extract. This potentiation was antagonized by cyproheptadine, a serotonin antagonist. This evidence of action mediated-through serotonergic system indicates that the extract may become a useful antidepressant. The speculation was tested on behavioral despaired-mice which was forced to swim. A weak antide-pressant effect compared to an antidepressant, imipramine, was observed.