1. The gastrointestinal motor stimulating action of the motilin analogue, KW-5139 (Leu13-motilin), was investigated both in the anaesthetized rabbit and in rabbit isolated smooth muscle tissues. 2. KW-5139 (0.3-10 micrograms kg-1, i.v.) produced motor stimulating actions in the gastric antrum, ileum and descending colon, the excitatory responses of which were initiated at the same time but declined with different time courses. The rank order of the excitatory response was: descending colon > or = gastric antrum >> ileum. 3. Atropine (1-3 mg kg-1, i.v.) or naloxone (1 mg kg-1, i.v.) completely suppressed the excitatory response to KW-5139 in the gastric antrum, but only partially attenuated that in the descending colon. This suggests that the mechanism of the excitatory response is different in the gastric antrum and the descending colon, and that cholinergic neural pathway is involved in the response of the gastric antrum. 4. KW-5139 (0.1 nM-1 microM) caused concentration-dependent contractions of the gastric antrum, duodenum, jejunum, ileum and the descending colon in vitro. In the rabbit intestine, the contractile response to KW-5139 was strongest in the duodenum and weakest in the ileum. 5. The contractile response to KW-5139 in the intestinal segments were not affected by tetrodotoxin, but were decreased by verapamil, or pretreatment with a high concentration of porcine motilin, confirming the involvement of motilin receptors in the response to KW-5139. 6. The present results suggest that the rabbit is a suitable species for the investigation of motilin on gut motility, because of the high responsiveness of the descending colon as well as the upper gastrointestinal tract.