Dimaprit, a highly selective H2-agonist, caused a multiphasic contraction of guinea-pig ileal segments and ileal myenteric plexus-longitudinal muscle preparations. The initial phase was characterized by a twitch which reached a maximum in 15 to 30 sec and was followed by a partial relaxation. The later phase was variable and consisted of a series of twitch responses or of a slowly developing contracture which sometimes was accompanied by oscillatory changes in tension. dose-response curves were generated for the initial response; for isolated ileal segments the EC50 was 5.1 +/- 1.8 micrometers (mean +/- S.D., N = 7) and the Hill coefficient was 1.1 +/- 0.2 and for longitudinal muscle strips the EC50 was 5.8 +/- 1.2 micrometer and the Hill coefficient was 1.2 +/- 0.1 (N = 7). Both the initial and secondary components of the contractile responses to dimaprit were prevented by 0.2 micron tetrodotoxin or 10 microns mefenamic acid and by the production of tachphylaxis to either substance P or serotonin. Scopolamine, 0.001 to 0.1 micron, insurmountably antagonized only the initial component of the response. Mepyramine (1.0 micrometer), hexamethonium (100 microns), bromolysergic acid (0.25 microns) and p-(imidazol-1-yl)phenyl (10 microns) were without effect on the response to dimaprit. The histamine H2-receptor antagonist, tiotidine, produced parallel dextral shifts in the dose-response curve for dimaprit. The apparent pA2 value for tiotidine was 7.65. The results suggest that dimaprit acts on H2-receptors located on myenteric plexus neurons to cause the release of contractile substances. The mediators of the contractile response are tentatively identified as acetylcholine, substance P, serotonin and a product(s) of the arachadonic acid cascade.