The enterotoxin E. coli STa induced fluid secretion in the rat jejunum, ileum and proximal colon in vivo that was greatly inhibited by the co-presence of luminal capsaicin, which is a specific neural toxin of afferent C fibres. The same dose of capsaicin had no effect on the fluid secretion activated by carbachol in the jejunum, ileum and proximal colon. Afferent C fibres appear to be involved in the activation by STa of fluid secretion in the rat intestine in vivo.