The motor responses of the jejunum and colon to stimulation of alpha 2-adrenoceptors by medetomidine and clonidine were investigated in four dogs. In fasting dogs, medetomidine, at a dose rate of 30 micrograms/kg i.v., disrupted the migrating myoelectric complex (MMC) pattern of the small intestine for about 2 h. Similar, but shorter-lasting effects were also induced by clonidine (30 micrograms/kg i.v.) on the jejunum. The administration of alpha 2-agonists inhibited colonic motility in fasting dogs, although medetomidine-induced inhibition was preceded by a short period of increased muscle tone. All these effects were reversed by the alpha 2-antagonists atipamezole (0.15 mg/kg i.v.) and yohimbine (0.20 mg/kg i.v.). In fed dogs, medetomidine (30 micrograms/kg i.v.) induced a strong increase of the tone on the proximal colon, while the activity of the medium and distal colon was completely suppressed. Yohimbine (0.50 mg/kg i.v.) immediately restored the activity of the colon and induced a propagated giant contraction and defaecation by the animal. These data confirm the importance of alpha 2-adrenergic receptors in the control of intestinal and colonic motility in the dog.