The role of peripheral and central opioid receptors in morphine-induced migrating myoelectric complexes (MMECs) was studied in conscious dogs implanted with silver-silver chloride electrodes. In normal fasted dogs morphine (100-200 micrograms/kg iv) initiated phase III of the MMEC in the duodenum. Once initiated the MMEC propagated distally. This effect of morphine was blocked by the opioid receptor antagonists naloxone (2 mg/kg iv) and N,N-diallylnormorphinium bromide (4 mg/kg iv). Higher doses of morphine (300-600 micrograms/kg iv) initiated phase III activity in fed dogs as early as 20 min after feeding, while lower doses (150 micrograms/kg iv) initiated phase III activity routinely when administered 100 min after feeding. In dogs with bilateral vagotomies and bilateral thoracolumbar sympathetic chain ganglionectomies, morphine (150 micrograms/kg iv) initiated phase III activity in the duodenum, which then migrated distally. This study demonstrates that morphine initiates phase III of the MMEC by acting through peripheral opioid receptors.