Substance P (SP) caused a concentration-dependent increase in short-circuit current of rat isolated colonic mucosal preparations (ED50 10 nM). The SP antagonist [D-Arg1,D-Pro2,D-Trp7,9, Leu11]SP (50 microM) did not increase short-circuit current. Tetrodotoxin (3.1 microM) reduced the effect of a maximum concentration of SP (300 nM). This reduction was increased when tetrodotoxin was given with [D-Arg1,D-Pro2,D-Trp7,9,Leu11]SP. Increases in short-circuit current produced by electrical field stimulation were not reduced by [D-Arg1,D-Pro2,D-Trp7,9,Leu11]SP. It is concluded that SP is not a transmitter at the neuroepithelial junction in rat colonic mucosa.