Binding studies with iodinated motilin revealed that in the small intestine motilin receptor density decreased aborally, disappeared in the caecum but returned in the colon and rectum. The highest density was in the distal colon (112 +/- /11 fmol/mg protein). The dissociation constant was the same in all regions (overall mean 1.10 +/- 0.22 nM). The ability of erythromycin-A (EM-A) and of two derivatives, EM-A N-oxide and EM-523, to displace motilin showed no difference between the tissues studied. Their order of potency was: motilin > EM-523 > EM-A > EM-A N-oxide. Proximal circular colonic smooth muscle strips showed maximal contractile responses towards motilin, EM-523 and EM-A of, respectively, 80 +/- 3%, 78 +/- 4% and 84 +/- 2% relative to the maximum obtained with acetylcholine. In proximal longitudinal muscle only a response of +/- 20% was obtained. Similar responses were obtained in the distal colon. The order of potency to induce contractions as reflected in the pED50 values was: motilin (8.03 +/- 0.1) > EM-523 (7.55 +/- 0.03) > EM-A (5.84 +/- 0.04) in proximal circular colon. The responses were not blocked by TTX (10(-6) M) or atropine (10(-6) M), but were reduced by verapamil (10(-6) M). The abundancy of motilin receptors in colonic smooth muscle, if applicable to other species, opens new perspectives for the therapeutic applications of macrolides with motilin agonist properties.