1. In the isolated guinea-pig taenia, application of X537A to the muscle caused a relaxation, while A23187 caused a sustained contraction. 2. Treatment with adrenergic blocking agents and tetrodotoxin had no effect on the relaxing response to X537A. 3. In the presence of a low concentration (10(-6) M) of X537A or under hypoxic conditions (bubbled with 95% N2: 5% CO2), the taenia lost the ability to respond to a high concentration (40 mM) of potassium with a sustained tonic contraction, although the rapid phasic contraction was still present. 4. When a low concentration of X537A was applied the shape of the contractile response to A23187 was changed from a sustained development of tension to an oscillatory tension which was also observed under hypoxia. 5. In tissues treated with glycerol for 3 weeks, neither X537A nor A23187 had any effect on the contractile response induced by elevation of calcium concentration (pCa = 4) in the presence of magnesium and ATP. 6. Exposure to a low concentration (10(-6) M) of X537A, or hypoxia, slightly yet significantly decreased the ATP content of the muscle. A high concentration (10(-5) M) of X537A markedly decreased the ATP content to about half normal. A23187 did not alter the ATP content. 7. Application of X537A did not alter the calcium content of the muscle and inhibited the A23187-induced increase in content. Under hypoxia, A23187 failed to increase the calcium content of the muscle. 8. The results indicate that, in contrast to A23187, X537A has a relaxing and metabolic inhibitory action on the guinea-pig taenia. The action of low concentrations of X537A resembled that of the hypoxia, indicating that X537A might exert its relaxing action, at least in part, by inhibition of aerobic energy metabolism of the muscle.