Background & Aims: Substance P, a member of the tachykinin family, is a prosecretory neuropeptide distributed widely throughout the enteric nervous system. Implicated in inflammatory states, its role in enterotoxigenic water and electrolyte secretion is unclear. We assessed the effect of substance P antagonists and neurokinin receptor antagonists on cholera toxin–, Escherichia coli heat-labile enterotoxin (LT)–, and heat-stable enterotoxin (STa)–induced water secretion in an in vivo rat jejunal perfusion model. Methods: Anesthetized adult male Wistar rats were pretreated with substance P antagonists (D-Pro 2 , D-Trp 7,9 , substance P, 0.1–3.0 mg/kg; or CP 96,345/4, 0.3–3 mg/kg) or neurokinin (NK)-1 (sendide, 1.0 mg/kg), NK-2 (GR83074, 1.0 mg/kg), or NK-3 ([Trp 7 ,βAla 8 ]NKA(4-10), 1.0 mg/kg) receptor antagonists. In a subgroup, extrinsic sensory afferents were ablated by pretreatment with capsaicin. Jejunal perfusion, with a plasma electrolyte solution containing a nonabsorbable marker, was undertaken after exposure to cholera toxin (25 μg), LT (25 μg), STa (200 μg/L), or saline. Results: Cholera toxin–induced water and electrolyte secretion was inhibited by the substance P antagonists and the NK-1 and NK-2 receptor antagonists, but not by the NK-3 receptor antagonist or by pretreatment with capsaicin. Neither LT- nor STa-induced secretions were affected by the pretreatments. Conclusions: Prosecretory pathways involving NK-1 and NK-2 receptors specifically mediate the actions of cholera toxin in the small intestine. GASTROENTEROLOGY 2000;119:1037-1044