Intraluminal release of serotonin (5-HT) was evaluated in relation to the interdigestive migrating complex (IMC) in the canine small intestine. Thirty-Vella loops (TVL) were constructed in the proximal jejunum of eight dogs. After recovery, the loops were continuously perfused with normal saline at 37 degrees C and changes in intraluminal pressure recorded. For each 10-min period, a motility index (MI) was calculated using computer-assisted planimetry and expressed in mm2/10 min. Serotonin contents in the effluent perfusates were measured by radioimmunoassay. A characteristic phasic motor activity was recognized in the TVL with periods of rest or minimal activity (MI = 406 +/- 45 mm2/10 min, Phase I) alternating with periods of vigorous activity (MI = 2082 +/- 134 mm2/10 min, Phases II and III). Serotonin was constantly found in the effluent perfusates in relatively high concentrations, but 5-HT levels did not vary significantly with intestinal activity; levels averaged 120 +/- 19 ng/ml during Phase I and 110 +/- 13 ng/ml during phases II and III. We conclude that intraluminal 5-HT plays no major role in the physiologic local regulation of the canine IMC in the small intestine.