Prostaglandin F2 alpha (PGF2 alpha) has recently been implicated in the pathogenesis of some diarrhea in man. PGF2 alpha has been shown to increase the smooth muscle contractile motility of some gastrointestinal muscles in vitro. The in vivo effects of PGF2 alpha on bowel smooth muscles are not clearly delineated. The aim of this study was to investigate the effect of PGF2 alpha on motility of the small and large intestine in the anesthetized dog. Six contractile force transducers were implanted to record contractions from both the circular and longitudinal muscles of the duodenum, ileum and colon. Blood pressure was monitored from the femoral artery and drug injections were made in the femoral vein. The i.v. administration of PGF2 alpha (1 microgram/kg/min) significantly stimulated duodenal circular muscle contractile frequency while depressing the longitudinal muscle contractile tone. In the ileum, PGF2 alpha markedly stimulated circular and longitudinal muscle contractions. In the circular ileum, tone was significantly increased while in the longitudinal ileum it was significantly decreased. In the colon PGF2 alpha did not significantly affect intestinal motility. These results suggest that the diarrheal effects of PGF2 alpha may be related to an effect on small bowel rather than on large bowel motility in the dog.