Male Sprague-Dawley rats weighing approximately 225 g and maintained under controlled lighting and temperature conditions, were used in this experiment. Animals were given morphine (10 mg/kg, i.p.) either at 10:00 hr or at 22:00 hr. One hour later, treated animals with their proper control were sacrificed and the mucosa of the stomach, duodenum, ileum and colon were separated and assayed for acetylcholinesterase (AChE) and choline acetyltransferase (ChAT) activities. Data obtained indicate that morphine administration during the light phase resulted in a significant increase (p less than 0.01) in AChE activity in the colon, with no significant effect on the stomach, duodenum or the ileum. There was also significant increase (p less than 0.05) in ChAT activity in the stomach and the duodenum as a result of morphine administration during the light phase, with no effect on other tissues studied. During the dark phase, there were no significant changes observed in AChE activity of all tissues examined following morphine administration. However, the determination of ChAT activity during the dark phase, following morphine treatment, showed a significant increase (p less than 0.01) in the duodenum and the ileum, with a significant decline (P less than 0.05) in the colon. Results obtained from this study indicate that gastrointestinal changes seen after morphine administration could be due to changes in the activity of the cholinergic enzymes and these changes may be diurnally controlled.