Seventeen human subjects fasted without electrolyte replacement for 3 days and hormone levels were measured before, during and after the fast. Immediate consequences of the fasting state in healthy human subjects include a marked increase in plasma cortisol. ACTH, beta-endorphin, beta-lipotrophic hormone, adrenaline, noradrenaline and dopamine. Levels of all these hormones were much greater on the first morning of the fast than in the post-prandial state, even though the plasma glucose level was no lower than that observed on the morning before the fast began. A clear fall in TSH and tri-iodothyronine (T3) levels was observed, but thyroxine levels did not change significantly. Insulin levels fell whereas proinsulin levels did not fall during the fast, though they did rise markedly upon re-feeding. An increase in GH levels was particularly apparent in male subjects, but was also seen in females when evening samples were compared. Pancreatic glucagon showed a modest rise during the fast, but fell again on refeeding; total glucagon also rose as the fast proceeded, but increased markedly upon re-feeding. Levels of gastrin and peptide YY remained low during the fast. Plasma electrolyte levels were unchanged. The following were closely correlated: cortisol with ACTH, T3 with log10 TSH, dopamine with noradrenaline, and (negatively, during the fast) pancreatic glucagon with glucose.