Associations between migrating myoelectric complexes (m.m.c.s) and peak plasma motilin concentrations were confirmed in the dog fasted 18 h and shown not to be present in pigs fasted 3-4 h. Infusions of both natural porcine and synthetic 13-Nle-motilin failed to induce m.m.c.s in the pig. It was confirmed that motilin infusions stimulated the premature appearance of m.m.c.s in the dog whether motilin remained within, or exceeded, its normal plasma values. Immunoneutralization by intravenous administration of rabbit antimotilin serum was without effect on naturally occurring m.m.c.s in the dog and the pig. In the dog, antimotilin serum blocked production of m.m.c.s by exogenous motilin for 7-10 d post-immunoneutralization. It is suggested that there are both: (i) species differences in associations of m.m.c.s and plasma motilin concentration, and (ii) an independence of m.m.c.s from plasma motilin even in the dog in which normally exogenous motilin can produce m.m.c.s.