Stimulation of the splenic sympathetic nerves in the dog and the cat has been shown to increase the outflow of prostaglandin from the spleen (Davies, Horton and Withrington 1967, Gilmore, Vane and Wyllie 1968). Recently prostaglandin E1 was found occasionally to reduce the release of noradrenaline (NA) caused by stimulation of the splenic nerves in the cat (Hedqvist and Brundin 1969). © 1969 Scandinavian Physiological Society