A biochemical study of the endogenous levels of serotonin (5-HT), noradrenaline (NA) and the activity of choline acetyltransferase (CAT) was carried out in the intestinal tract of the rat. High levels of 5-HT and NA were detected in the caecum and the colon. These anatomical regions also presented the highest activity of CAT. Similar activities of CAT were detected, after dissection, in the mucosa and the muscular layers containing the enteric plexuses. During the day-night cycle, 5-HT and NA amounts showed significant variations as a function of time. Treatment with pargyline (75 mg kg(?1)), a monoamine oxidase inhibitor, resulted in an increase in 5-HT content with parallel modifications in CAT activity. In spite of an important decrease in 5-HT endogenous level in the caecum of rats pretreated with parachlorophenylalanine (300 mg kg(?1)), no significant change in CAT activity was detected whatever was the duration of the treatment. ?-Methylparatyrosine (100 mg kg(?1)), known to block the synthesis of NA, did not affect the CAT activity in the caecum.