Rat intestinal transit was determined 30, 60, 150 and 270 min after morphine sulfate or saline given either subcutaneously (s.c.), intraduodenally (i.d.) or intracerebroventricularly (i.c.v.). In addition, two popular methods of quantifying the progression of a radioactive marker along the bowel, the intercept and geometric center methods, were compared. Morphine (5 mg/kg) given s.c. and i.d. produced two distinct periods of reduced intestinal transit, an early phase at 30 min and a late phase at 150 min. Neither phase of the response was influenced by transection of the vagus nerve. The late phase was not affected by cannulation of the bile duct but was completely extinguished when animals were treated with saline instead of the ricinoleic acid, a cathartic agent used in the original experimental protocol. Given i.c.v., morphine (30 micrograms, total dose) inhibited intestinal transit 30 and 60 min after administration. The results indicate that the temporal effects of morphine on rat intestinal transit differ after central and peripheral administration, and in animals treated and not treated with a cathartic agent. Both methods of calculating intestinal transit produced similar statistical results.