In spite of numerous studies of short-term (less than 30 days) fasts, current knowledge of prolonged fasting, the mechanisms of adaptation, and the causes of death are limited.A healthy 48-year-old man fasted voluntarily, on water only, for 54 days, during which time he offered himself as an experimental subject. Numerous metabolic parameters were measured, and his mental state was evaluated. Those parameters not generally measured in previously published fasts are thyrotropin-releasing hormone levels (TRH), reverse T(3), and leuenkephalin levels.Without experiencing any major difficulties, the subject completed a 20-mile jog and a 60-mile walk eight days after the completion of his water fast, during which time he "weaned" himself on juices and a mixture of vitamins, minerals, and herbs.