The ingestion of a meal in nonruminants disrupts the cycling of migrating myoelectric complexes for several hours. We investigated the initiation of phase III activity during the postprandial state by morphine. Small intestinal recordings were made from 5 dogs by surgically implanted electrodes. Morphine boluses (5-400 micrograms/kg) were given during the fasted state and after a meal. Morphine initiated premature phase III activity in the fasted state and it also initiated phase III activity in the postprandial state. Motilin did not initiate phase III activity in the postprandial state. The mean durations of morphine-initiated phase III activity in the fasted state and in the postprandial state were not significantly different from that of spontaneous phase III activity; however, morphine-initiated phase III activity in both the fasted and fed states migrated faster than spontaneous phase III activity in the proximal half of small intestine but not in the distal half. The latent period for the initiation of phase III activity was significantly greater 20-40 min after the meal than 2 h after the meal. The minimum dose of morphine required to initiate phase III activity in the fed state decreased progressively after the initial increase until it reached fasted levels 7-10 h after the meal. Spontaneous phase III activity appeared after this period. We conclude that (a) morphine temporarily overcomes the disruption of migrating myoelectric complex cycling after a meal; (b) morphine acts at different sites than motilin to initiate phase III activity; (c) the increased refractoriness of migrating myoelectric complex cycling mechanisms after a meal may play a role in the disruption of migrating myoelectric complex cycling.