In an attempt to explain reverse T3 changes during protein supplemented semistarvation, reverse T3 (rT3) and T3 changes have been evaluated in regard of bw changes and changes of nutrient combustion. Significant (p less than 0.001) bw and body mass decrease was characterized by a significant (p less than 0.01) increase in rT3 levels, whereas no significant decrease of T3 could be found. During semistarvation carbohydrate combustion decreased from 34 to 13% where lipid combustion rate increased from 51 to 72%. No correlation could be found between delta rT3 and changes in bw, BMR or oxygen consumption. A significant correlation (p less than 0.005) exists between delta rT3 and delta % lipid combustion, while a negative correlation (p less than 0.02) was present between delta rT3 and delta % protein combustion. No correlation exists between delta rT3 and carbohydrate combustion. Although no definite clue could be found for the rT3 increase during semistarvation, a combined effect of lipolysis and lean mass preservation is suggested.