Five 5 to 6 month old horses were surgically prepared with silver electrodes sutured to the serosa of gastric antrum, duodenum and proximal portions of the jejunum. Normal migrating motility complex (MMC) periodicity was determined during daytime hours in horses that were fed and horses from which food was withheld for 24 hours. Periodicity was defined as time span from the end of one period of regular spike activity (RSA) to the end of the next RSA in the MMC. The periodicity was 120.5 +/- 9.5 (SEM) minutes in horses from which food was withheld, and was 125.7 +/- 20.3 minutes in horses fed hay free choice. Coincident with each duodenal RSA, antral spike activity ceased. Xylazine (0.25 and 0.5 mg/kg), given IV during the period of intermittent spike activity of the MMC to either fed or unfed horses induced, within 2 minutes, a RSA complex in the duodenum that migrated to the proximal portion of the jejunum. This was followed by a period of no spike activity of normal duration, which proceeded on to a period of intermittent spike activity of varying duration to complete the MMC cycle. Pretreatment IV administration of an alpha 2-adrenergic antagonist, tolazoline (1 mg/kg) also provoked a RSA complex, but blocked the xylazine effect. The results indicated that xylazine resets the duodenal MMC in the horse, but does not seriously disrupt proximal gastrointestinal tract motility, and that control of MMC periodicity in this region probably involves more than alpha 2-adrenergic receptors.