Twelve rats were subjected to paradoxical sleep deprivation (PSD) by the disk apparatus. All PSD rats died or were sacrificed when death seemed imminent within 16-54 days. No anatomical cause of death was identified. All PSD rats showed a debilitated appearance, lesions on their tails and paws, and weight loss in spite of increased food intake. Their yoked control (PSC) rats remained healthy. Since dehydration was ruled out and several measures indicated normal or accelerated use of nutrients, the food-weight changes in PSD rats were attributed to increased energy expenditure (EE). The measurement of EE, based upon caloric value of food, weight, and wastes, indicated that all PSD rats increased EE, with mean levels reaching more than twice baseline values. All of these changes had been observed in rats deprived totally of sleep; the major difference was that they developed more slowly in PSD rats.