Gastrointestinal myoelectric activity was studied during intravenous infusion of bombesin and bombesin-like peptides in conscious dogs with electrodes chronically implanted at different levels between the stomach and the rectum. The peptides used were bombesin, the COOH-terminal hepta- octa-, and nonapeptide of bombesin, and litorin, a new natural peptide isolated from the skin of Litoria aurea. Bombesin significantly increased the frquency of pacesetter potentials (PP) in tha antrum, duodenum, jejunum, and ileum. In the duodenum and jejunum the increase of PP frequency showed linear correlation with the reduction of PP amplitude. The propagation velocity of PP was clearly reduced. Spikes were not affected in the antrum and ileum, whereas they were abolished in the duodenum and jejunum. In the duodenum the increase of PP frequency and the slowing down of propagation velocity was followed by the loss of PP phase lock and the appearance of a characteristic electric pattern, consisting of an irregular sequence of small and slow potentials ("electric disorganization"). The mechanical counterpart was the disappearance of intraluminal pressure activity. In the colon the effect of bombesin on electric activity was not consistent. Neither the COOH-terminal heptapeptide nor the octapeptide of bombesin showed a significant effect on myoelectric activity, whereas the effect of COOH-terminal-nonapeptide and litorin was similar to that of bombesin. Thus, the characteristic electric changes of PP produced by bombesin appear to be related to the sequence of the nine amino acids in the COOH-terminal residue of the bombesin molecule.