In organ bath experiments, pyrogallol (30-100 microM), and to a lesser extent L-cysteine (3 microM), reduced nitric oxide (NO)-induced relaxation, but not the NO-mediated non-adrenergic non-cholinergic (NANC) relaxation elicited by field stimulation of the canine ileocolonic junction. In contrast, in a superfusion bioassay, pyrogallol (10-30 microM) and L-cysteine (1 microM) inhibited the biological activity of both the transferable nitrergic factor released from the canine ileocolonic junction in response to NANC nerve stimulation and NO, but not that of S-nitroso-L-cysteine, S-nitrosoglutathione or S-nitroso-N-acetyl-D,L-penicillamine. Based on the bioassay experiments, it is concluded that the nitrergic NANC neurotransmitter in the canine ileocolonic junction behaves pharmacologically like NO.