Brain serotonin level was increased by supplementing the diet in two groups of rats. They were first exposed to conditions of shock, either escapable or inescapable. They were then placed in a cool water bath, and the response latency to successful escape was noted. Two other groups fed normal diets were similarly examined. Tryptophan effects of hypothermia were shown to interact with the analgesic effects of serotonin and to the tolerance of the noxious water bath. The results are discussed in relation to brain serotonin effects on escape following shock pretreatment ("learned helplessness" and its relevance to clinical depression.