Reserpin, a new, highly active alcaloid from Rauwolfia serpentina benth., shows a very marked hypnotic effect and lowers the blood pressure. Reserpin inhibits pressor responses evoked by electrical stimulation of the afferent vagus and sciatic nerves, and inhibits the pressor reflex of the carotid sinus. It does not inhibit pressor responses evoked by electrical stimulation of the splanchnic nerves. Reserpin has no peripheral sympathicolytic and no ganglionic blocking activity; the action of sympathicomimetic substances, e.g. adrenaline, nor-adrenaline, ephedrine, is enhanced. It does not potentiate the acetylcholine effect. The blood pressure effect is not inhibited by atropine and by cutting the vagi and depressor nerves. Reserpin stimulates peristalsis. The neuro-muscular transmission is not impaired. There is evidence, that central portions of the sympathetic system are inhibited by Reserpin. © 1953 Birkhäuser Verlag.