The effects of imidazole and phentolamine on the relaxant responses of the isolated guinea-pig taenia coli to transrural stimulation (1 msec, 5 pulses/sec) and to ATP (10^-6-10^-5g/ml) were studied. The relaxation produced by ATP was entirely abolished by imidazole (3.5×10^-3g/ml) or phentolamine (5×10^-5g/ml), though the relaxation due to transmural stimulation remained unimpaired. These results are rather controversial to the concept that the transmitter substance of the non-adrenergic inhibitory nerves in the gastrointestinal tract is ATP.