The metabolic response to different periods of fasting, ranging from 3 to 7 days, was studied in eight nonobese subjects. Blood ketones rose progressively to 5.07 nmol/l with the prolongation of the fast. The concentrations of glucose and alanine decreased significantly. Valine and leucine rose during fasting but the rise was not statistically significant. Serum triiodothyronine (T3) and insulin decreased, while the urinary excretion of 17-hydroxycorticosteroids (OHCS) doubled. Urea nitrogen decreased in those who fasted 5 to 7 days. Total urinary nitrogen excretion did not change significantly. Excretion of 3-methylhistidine (3-MH) rose during fasting and in some subjects more than doubled when compared with the fed state. The discrepancy between the loss of body protein (calculated by comparing excretion of urinary nitrogen) and the loss of muscle protein (calculated from the excretion of 3-MH) suggests a high rate of recycling of nitrogen during fasting in our subjects.