ABSTRACT The possible influence of a 48 h period of sleep deprivation on the serum concentrations of total and unbound 3,5,3',5'-thyroxine (T4) and 3,5,3'- triiodothyronine (T3), and of total 3,3',5'-triiodothyronine (reverse T3, rT3) was studied in 12 young male volunteers. In addition, plasma concentrations of TSH and the urinary excretion of noradrenaline and adrenaline were examined. Both total S-T4, S-T3 and S-rT3, and unbound S-T4 and S-T3 (unbound rT3 was not calculated) were significantly enhanced following sleep deprivation. P-TSH showed a numerical but statistically not significant increase, while the catecholamines remained unaltered. The results suggest that sleep deprivation provokes enhanced secretion of thyroid hormone(s), supposedly via increased secretion of TSH.