The action of substance P (SP) on mucosal ion transport has been investigated in the guinea-pig small intestine. Segments of intestine were dissected free of external muscle and myenteric plexus and mounted in Ussing chambers. Short-circuit current (Isc) was measured as an indication of net ion transport across the tissue. SP (greater than 10(-10) M) added to the submucosal side of the tissue caused a transient increase in Isc. Tetrodotoxin (TTX, 10(-7) M) decreased the maximum SP response to 11% of the control value. TTX completely inhibited the response to electrical field stimulation but had no effect on Isc increases due to carbachol or theophylline. In the presence of hyoscine (10(-7) M) the SP response was reduced to 42% of the control value, but hyoscine had no effect on the TTX-resistant SP response. Mepyramine (10(-6)M) had no significant effect on the SP response. These results suggest that SP alters mucosal ion transport by stimulation of cholinergic and non-cholinergic nerves in the mucosa-submucosa. A small part of the SP response appears to be due to a direct action on epithelial cells. The SP antagonist (D-Arg1, D-Pro2, D-Trp7,9, Leu11)-SP decreased the magnitude of the TTX-resistant SP response, and caused a decrease of similar magnitude in the total SP response. These results imply that the major component of the SP response, which is due to an action on neurons, is unaffected by this antagonist. It is concluded that the SP receptors on epithelial cells are blocked by the antagonist and are different to the SP receptors on submucous neurons, which are not blocked by the antagonist.