Using the completely denervated iris of the cat as an indicator of circulating autonomic substances, it was found that constriction of the pupil ensued from 35 to 70 seconds following the intravenous administration of 1.0 cc of whole human blood. The blood of rabbit, guinea pig, mouse, monkey, hog and hemolyzed erythrocytes of cat also produced a constriction of the completely denervated iris of the cat. Identical amounts of blood from dog, sheep and ox were without significant effect on the same preparation. The most conspicuous reactions were obtained with rabbit's blood. Large quantities of dog, ox, or sheep blood sometimes produced a slight and slow constriction of the cat's denervated iris.Other effects caused by the intravenous injection of blood into the cat were a fall in blood pressure, as shown by Abramson, et al.,1 and Fuzii,2 decrease in heart rate, relaxation of anal and urinary sphincters and hypersalivation, all parasympathomimetic manifestations.The vaso depressor and miotic propertiesof t...