Morphine inhibits gastrointestinal transit in the rat primarily by impairing propulsive activity of the small intestine.

The effect of a single dose of morphine on small intestinal transit was studied in fasted rats by feeding a charcoal test meal either by stomach tube to normal animals or through a duodenal cannula to chronically implanted rats. The percentage of small intestine traversed by charcoal in 5 min was reduced by morphine (0.7 mg/kg s.c.) given 5 min before the meal to a similar extent in normal and implanted rats. In contrast administration of atropine (0.8 mg/kg s.c.) 30 min before the meal reduced charcoal transit only in normal rats fed the test meal by stomach tube. Inhibition of gastrointestinal transit in the rat by systemically administered morphine is mainly due to impaired small intestinal propulsion and does not depend to any significant extent on a direct action on the stomach delaying gastric emptying.