1. Sodium succinate was found to have a definite antidotal effect on the mescaline psychosis in each of 12 subjects.2. The effect varied greatly in different subjects. Moreover, the effect was rather transient and there was little shortening of the total duration of mescaline effect.3. The most plausible hypothesis of the succinate effect is that succinate provides a substrate for oxidation in the brain tissues when the oxidation of other substrates, e.g., glucose, lactate and pyruvate has been depressed by the mescaline.