The effect of D- and L-isomers of tryptophan and phenylalanine on fasting motor activity of canine jejunum were investigated. Only L-tryptophan had any effect on jejunal motility. The concentration of L-tryptophan required to elicit a motor effect in 50% of animals was estimated at 28 mM. 50 mM-L-tryptophan stimulated jejunal motility after a latency of 4.5 +/- 0.7 min. The pattern of motility was similar to that evoked by feeding but continued for only 41 +/- 5 min and failed to disrupt the normal timing of the interdigestive migrating motor complex (m.m.c.). The duration of the subsequent phase II of the complex was, however, significantly reduced. Multiple infusion of L-tryptophan also failed to disrupt the timing of the m.m.c. These data are consistent with the hypothesis that specific breakdown components of protein digestion are implicated in the stimulation of postprandial motor activity. The mechanisms involved in the motor response to L-tryptophan are discussed.