In open-chest, anesthetized dogs, the effects of blocking the neurona uptake mechanisms for norepinephrine (NE) were studied on the inotropic and chronotropic responses to cardiac sympathetic nerve stimulation. Cocaine (COC), a standard neuronal uptake blocking agent, did not significantly potentiate the ino tropic and chlorpheniramine (CHL) and tripelennamine (TRI), which are also potent neuronal uptake blocking agents, only slightly potentiated the inotropic responses were significantly prolonged after either of these antihistaminic drugs had been given. It is suggested that the absence of appreciable potentiation of the inotropic and chronotropic responses to sympathetic neural stimulation in vivo is not a property peculiar to specific neuronal uptake blocking drugs, such as COC and desipramine (DMI), but is probably a general characteristic of such agents.