Metoclopramide (Mcp) is known to facilitate gastrointestinal emptying in vivo and to stimulate various isolated intestinal muscle preparations. On the guinea pig ileum, taenia coli, rabbit ileum and rat duodenum, Mcp increased the tone and responses to acetylcholine, carbachol and nicotine; had no effect on responses to histamine, potassium chloride and prostaglandin E1; decreased responses to 5-hydroxytryptamine (5-HT). Atropine, methysergide, morphine, and tetrodotoxin, alone or in combination, partially blocked the stimulatory responses to Mcp, but hexamethonium, mepyramine and indomethacin did not. Mcp (1.0 muM) lowered the threshold for elicitation of the peristaltic reflex to a sub-threshold intraluminal pressure (2.5 cm water), facilitated the peristaltic response to threshold pressures (3-4 cm water) and restored the reflex in fatigued preparations, but not that depressed by cooling to 24 degrees C. During block of peristalsis by atropine, hexamethonium or methysergide (applied serosally) 5-HT (0.25 muM) but not Mcp (1.0 muM)) effectively restored the peristaltic reflex, but neither antagonized the inhibition by morphine or procaine acting serosally. However, Mcp (1.0 muM) re-established peristalsis inhibited by a high concentration of 5-HT ((4 X 10 muM). These results do not support the hypotheses that the stimulatory action of Mcp is entirely dependent on either peripheral sensitization of muscarinic receptors or an action on tryptaminergic mechanisms but are consistent with our previous conclusion that an additional component may be a blockade of some intrinsic inhibitory (possibly purinergic) substance normally restraining intestinal motility or tone.