1. Prostaglandins E(1) and E(2) affected intestinal activity both in vitro and in vivo.2. Serosal application of prostaglandin to guinea-pig isolated ileum stimulated the longitudinal muscle but reduced peristaltic contractions of the circular muscle and the propulsion of fluid through the gut. Intraluminal application had little effect.3. Injection of prostaglandin into the bloodstream of anaesthetized rats stimulated the longitudinal muscle of the ileum and increased the intraluminal pressure. A similar response sometimes occurred in the guinea-pig, but in general the effect was variable.4. Release of prostaglandin in the gut wall, but probably not into the blood or into the lumen of the gut, may play a part in controlling intestinal motility.