The action and mechanism of action of cholecystokinin octapeptide (CCK-8) and pentagastrin on isolated segments of guinea-pig duodenum were examined using contractility studies and by intracellular recordings made from smooth muscle cells. Both CCK-8 and pentagastrin caused an excitatory contractile response. The threshold concentration ranged from 5 X 10(-11) to 10(-9) M for CCK-8 and 5 X 10(-10) to 10(-8) M for pentagastrin. The excitatory response was abolished by tetrodotoxin (3.1 X 10(-6) M) and atropine (1.5 X 10(-6) M) and inhibited by d-tubocurarine (up to 2.9 X 10(-5) M). In the presence of atropine a proportion of preparations relaxed in response to CCK-8 (nineteen of thirty-one) and pentagastrin (thirteen of seventeen). This response was only seen at high concentrations of the peptides (10(-8)-10(-7) M) and was abolished by tetrodotoxin (3 X 10(-6) M). Intracellular recordings from duodenal smooth muscle revealed multiple excitatory junction potentials (e.j.p.s) in response to CCK-8 and to pentagastrin. These e.j.p.s were identical to those evoked by transmural nerve stimulation and were abolished by atropine (1.5 X 10(-7) M) and by tetrodotoxin (3 X 10(-6) M). Inhibitory junction potentials (i.j.p.s) were not recorded in response to the peptides except on one occasion. It is suggested that CCK-8 and pentagastrin cause an increase in duodenal motility by the selective activation of excitatory pathways in the enteric nervous system.