This review covers three areas in which dietary components may influence brain serotonin synthesis, mood, and behavior. In the first, tryptophan-deficient amino acid mixtures have been used to lower brain tryptophan and serotonin in normal human subjects for experimental purposes. Results suggest that low serotonin can cause lowered mood and increased aggression, but that it is probably not involved in any simple way in carbohydrate craving. Low serotonin levels can block the analgesic effect of morphine in humans. The second area concerns deficiencies of folic acid, which can cause low brain serotonin and lowered mood. Folate supplements may be useful in some depressed patients. The third area concerns the effect of carbohydrate meals. Although it is well established that carbohydrate meals raise brain tryptophan and serotonin in the rat, and that protein meals lower them, any effects of carbohydrate or protein meals on human brain serotonin are likely to be negligible under most circumstances. Carbohydrate meals can have definite effects on mood and behavior in humans, and there are a variety of mechanisms, other than alterations in brain serotonin, that might mediate these effects.