The in vitro spasmolytic activity of glycerol trinitrate was measured on the KCl-contraction of aorta strips from the rabbit. In the presence of sulphobromophthalein, a known inhibitor of glutathione S-transferase, the dose-activity curve for the nitrate was displaced to the right. Much smaller displacements were obtained with the control spasmolytic substances--papaverine and S-nitroso-N-acetylpenicillamine. It was confirmed that sulphobromophthalein inhibits glutathione S-transferase activity in aorta homogenates. Aorta extracts did not detectably catalyze the reaction between glutathione and sulphobromophthalein and the glutathione level was not decreased by treating the intact aorta with sulphobromophthalein. It is concluded that sulphobromophthalein acts as a specific antagonist of the spasmolytic activity of glycerol trinitrate, probably as a result of its inhibition of glutathione S-transferase. It thus seems probable that glutathione and glutathione S-transferase are involved in the pharmacological activation of the organic nitrates.