The substance P (SP)-like peptides present in the intestine of the cod, Gadus morhua, were studied by immunochemical techniques. The amount of SP-like material in extracts of the intestine was quantified using radioimmunoassay (RIA). Immunohistochemistry indicated that the SP-like material detected in nerve fibres and endocrine cells of the intestine is more closely related to SP than to the other tachykinins used in the study. The excitatory response of SP on the vascularly perfused cod intestine was reduced or abolished by treatment with atropine, methysergide or tetrodotoxin. The results indicate that SP acts partly via cholinergic and serotonergic neurons and partly directly on the smooth muscle cells of the intestine. The effect of SP was compared to the effects of 5 other tachykinins. No great difference was found between the potencies of the tachykinins, suggesting that the receptors present are most similar to the SP-P type of receptor.