Prostaglandins (PGE1, E2, F1alpha and F2alpha) have been tested on the peristaltic reflex in isolated segments of guinea-pig ileum and colon using simultaneous recordings of fluid propulsion and longitudinal and circular muscle activity. Propulsion and circular muscle peristaltic activity were increased by serosally applied PGF compounds in the ileum and PGE or PGF compounds in the colon following initial contraction of the longitudinal muscle. This is consistent with a role for prostaglandins in peristalsis. Mucosally applied PGF compounds has no significant effect.