We investigated the characteristics of spontaneously and pharmacologically induced giant migrating contractions (GMC) in the small intestine of conscious dogs. Myoelectric and contractile activities were recorded from eight sites, 40-70 cm apart, by surgically implanted bipolar electrodes and strain-gauge transducers, respectively. In addition to the usual phasic contractions in the fasted state, we observed spontaneous GMCs that generally originated in the mid or distal small intestine and migrated caudad. These contractions occurred infrequently and irregularly. The amplitude and duration of these giant contractions were severalfold larger than those of usual phasic contractions during phase III activity. The giant contractions migrated caudad at a velocity of approximately 1 cm/s. These contractions were preceded and followed by a period of inhibition of spontaneous motor activity. Some dogs whined and showed restlessness when giant contractions originated in the mid to upper small intestine. Intravenous morphine at 20, 50, 100, and 200 micrograms.kg-1 .h-1 or gastric instillation of 100 ml of 60% vinegar solution also initiated GMCs whose characteristics were similar to those of spontaneous GMCs. The GMC in response to these stimuli originated at more proximal sites than the spontaneous giant contractions. The GMC in response to morphine infusions occurred repeatedly, but in response to vinegar they occurred only once as a burst of contractions. These agents did not induce GMCs in the fed state. In most cases, the upstroke of a giant contraction was associated with a brief burst of electrical response activity. In some cases the burst of response activity lasted for the entire duration of giant contractions.