5-Hydroxytryptophan (5-HTP) caused a prolonged increase of respiratory (phrenic nerve) activity when given into the third ventricle of paralyzed, vagotomized and glomectomized cats. In contrast, intravenous 5-HTP always led to a prolonged inhibition. These findings must be due to different modes of action of the same drug given by different routes and illustrates that caution must be used in interpreting experimental results following administration of exogenous agents.