The regulation by gastrointestinal polypeptide hormones of contraction and relaxation of functionally isolated smooth muscle cells from gastric antrum of the rabbit has been investigated. Gastrin, cholecystokinin (CCK-8) and motilin induced a rapid contraction of isolated cells: significant response occurred within a 5-sec incubation with these peptides and maximal response (40% decrease in cell length) after 30 sec. A higher sensitivity of smooth muscle cells to gastrin and CCK-8 than to motilin stimulations was demonstrated (EC50 = 10 pM for both gastrin and CCK-8 and EC50 = 1 nM for motilin). The minimal gastrin fragment required to get full contraction was the C-terminal pentapeptide amide common to gastrin and CCK. Proglumide inhibited gastrin- or CCK-8- but not motilin-induced contractions with an IC50 of 50 microM. contraction induced by gastrin and motilin required normal levels of extracellular calcium, whereas that due to CCK-8 seemed to be independent of extracellular calcium. Vasoactive intestinal polypeptide (VIP) caused a relaxation of smooth muscle cells maximally contracted by carbachol or CCK-8 or gastrin (EC50 = 2.2 nM) with a parallel increase in intracellular cAMP content.