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1. The response of the smooth muscle of the guinea-pig taenia coli to adenosine triphosphate (ATP) and related substances was compared with that to noradrenaline and dopamine. The effect of these substances on the inhibitory potential was investigated.

2. ATP in concentration of 10^-6-10^-5 M suppressed the spontaneous spike and relaxed the preparation. ATP in a concentration higher than 10^-4 M hyperpolarized the membrane, reduced the membrane resistance and decreased the inhibitory potential.

3. Effects of noradrenaline (5 x 10^-6 M) and of dopamine (5 x 10^-4 M) were similar to those of ATP (10^-3 M). However, the effects of catecholamines were reduced in Cl-deficient solution and blocked by phentolamine (3·5 x 10^-6 M), while the effects of ATP were unaffected.

4. A high concentration of phentolamine (3·5 x 10^-4 M) almost abolished the inhibitory potential as well as the effects of ATP. The inhibitory potential and the effect of high concentration of ATP (10^-3 M) were not impaired by imidazole (50 mM).

5. Adenosine diphosphate, adenosine monophosphate and adenosine produced effects essentially similar to those of ATP. However, their potency was less compared with ATP.

6. These electrophysiological observations support the hypothesis that ATP may be the transmitter of the non-adrenergic inhibitory nerves in the taenia coli. No evidence against the hypothesis was obtained.