Tolonidine, 2(2-chloro-p-toluidino)-2-imidazoline-nitrate, is a substance chemically related to clonidine. In the anesthetized dog, tolonidine administered i.v. decreased the amplitude of ventricular contractions, reduced aortic blood flow and increased peripheral vascular resistances. In the bivagotomized pithed rat, tolonidine induced a long-lasting increase in blood pressure with no secondary hypotension, thus suggesting peripheral sympathomimetic properties, however, contractions of seminal vesicles in vitro were not obtained. The product proved to have no peripheral sympatholytic or parasympatholytic properties.