We studied the effect of motilin on myoelectric activity of the sphincter of Oddi (SO) and upper gastrointestinal tract in conscious opposums. In 17 animals, bipolar electrodes were implanted on the gastric antrum, SO, duodenum, and jejunum. Subsequent 8-h recordings reconfirmed our previous findings that SO spike burst rate changed with interdigestive cycles of the gastrointestinal migrating myoelectric complex (MMC), becoming maximal during passage of phase III activity through the duodenum. In eight animals, peak motilin levels were shown to occur concurrently with maximal SO spike burst rate and MMC phase III activity in the duodenum. Motilin infusion (0.3 and 0.9 micrograms X kg-1 X h-1), given for 30-60 min starting 10 min after duodenal phase III, elicited premature MMC activity that originated in the stomach. Maximal SO activity occurred coincident with passage of premature phase III activity through the duodenum. Pulse intravenous doses of motilin (25-1,600 ng/kg) generally caused an immediate increase in spike burst activity in the gastric antrum, duodenum, and SO that lasted 3-5 min and was often followed by a premature MMC, usually starting in the antrum and progressing through the duodenum and jejunum. Increases in SO spike burst rate also occurred concurrent with motilin-induced, premature duodenal phase III. Motilin given at 5-60% of the duodenal MMC cycle length elicited premature MMCs at 10-60% of the cycle, but no premature MMCs were elicited by any of the motilin doses at the 5% intervals.(ABSTRACT TRUNCATED AT 250 WORDS)