Ten days of total energy deprivation evoked the following endocrine changes in 12 healthy, normal-weight males: early and marked reductions and increments in the blood levels of T3 and reverse T3, respectively, with rapid returns to pre-starvation levels after refeeding; a slight and late decrease in the blood levels of T4; a minute reduction of the blood levels of TSH; a pronounced increase in the blood levels of growth hormone, but a return towards pre-exposure levels even before discontinuation of starving; a minor and gradual enhancement of the blood levels of cortisol, and an increase in nocturnal urinary adrenaline excretion. It is assumed that these changes reflect a complex regulatory mechanism, the purpose of which is to secure adequate energy supply to vital organs.