The effect of human alpha-calcitonin gene-related peptide (CGRP) on epithelial ion transport was investigated in guinea pig distal colon set up in Ussing flux chambers. Addition of CGRP to the serosal bathing solution evoked a dose-dependent increase in short-circuit current in whole-thickness tissues with intact myenteric and submucosal ganglia, but not in whole-thickness preparations when neural connections between myenteric and submucosal ganglia were severed, nor in sheets of submucosa/mucosa with intact submucosal ganglia. The effects of CGRP were nearly abolished in chloride-free solutions or after treatment with furosemide. Tetrodotoxin and hexamethonium abolished the effects of CGRP on basal short-circuit current whereas atropine did not. CGRP enhanced neurally evoked chloride secretion both in whole thickness and submucosa/mucosa preparations, but the effect in the latter was considerably smaller. These observations suggest that CGRP stimulates chloride secretion primarily by activating myenteric neurons that project either to submucosal ganglia or to the mucosa of the guinea pig distal colon. Furthermore, CGRP appears to have a greater effect on excitability of myenteric neurons than submucosal neurons.