The participation of cholinergic mechanisms in cholecystokinin octapeptide (CCKOP) action on canine gall bladder was studied in vivo and in vitro, using three different experimental conditions. In vitro the responses of canine gall bladder smooth muscle to CCKOP (0.01 to 10 nm) were insensitive to atropine (1 to 10 microM) and tetrodotoxin (3 microM). When gall bladder muscle preparations were contracted by field electrical stimulation (0.7 ms, 40 Hz) CCKOP (0.001 to 0.1 nM) enhanced these contractions while atropine (1 microM) abolished them. This suggests that CCKOP is able to influence acetylcholine (ACH)-release from intrinsic cholinergic nerve terminals. In vivo the responses of canine gall bladder smooth muscle to CCKOP (1 to 10 ng/kg i.v.) were reduced and even abolished by atropine (10 to 50 micrograms/kg i.v.) and hexamethonium (0.5 to 3 mg/kg i.v.). The results suggest the participation of at least two mechanisms in CCKOP action on canine gall bladder motility: a direct action on smooth muscle cells, insensitive to atropine or tetrodotoxin, and an indirect action, which is dependent on pre- and post ganglionic cholinergic pathways.