Using 85Sr-labelled microspheres as nonabsorbable markers, the effect of age on gastrointestinal propulsion motility was determined in conscious rats. There was no significant correlation between the gastric emptying rate or the propulsion motility of the small intestine and the age of the rats. However, the transit time of the intestinal content in the caecum in adult (17- to 20-month-old) rats was about fivefold the value found in weanling (19-day-old) rats. Also the transit time of the large intestine was found to be prolonged by about 70% during the life span investigated.