Field stimulation of intrinsic nerves (40 V, 0.5 ms, 1-5 pps for 3-10 min) or intraarterial administration of motilin (10(-11)-10(-8 mol) caused contractions of the canine small intestine that were partially resistant to atropine and abolished by tetrodotoxin. Naloxone (10 micrograms ia or 200 micrograms/kg iv) in selective doses (10(-10)-10(-9) mol) that left responses to intra-arterial acetylcholine unaffected reduced responses to field stimulation or motilin in the ileum in the absence of atropine or residual responses in the presence of atropine. Results in the jejunum were similar except that selective doses of naloxone (10 micrograms ia or 200 micrograms/kg iv) increased the sensitivity to motilin in the absence of atropine. Higher doses of naloxone decreased these responses. In common with naloxone, tachyphylaxis to methionine-enkephalin (Met-Enk) reduced the responses to field stimulation or motilin before or after atropine but did not affect the responses to acetylcholine before or the responses to field stimulation of muscle after atropine. These findings are consistent with the hypothesis that Met-Enk may be a noncholinergic, excitatory transmitter released by field stimulation of nerves or intra-arterial motilin in the canine intestine.