The effect of the infusion of mixed amino acids with glucose and fat on metabolic rate (MR) in sepsis (S) and in nonseptic trauma (NS) was investigated. The results demonstrated that diet-induced thermogenesis (specific dynamic action) controlled in S and in NS 25% and 18% of the MR variability, respectively (r2 = .25 and .18, p less than .01 for both). Also, diet-induced thermogenesis represented a quantitatively relevant portion of the total MR. There was a larger thermogenic effect of amino acids in S with respect to NS (22.3 vs. 5.7 cal/mg N, p less than .01) with a concomitantly increased thermogenic effect of fat. The results suggest the importance of recognizing the thermogenic effect of administered nutrients as an important determinant of MR during calorimetric measurements. The finding of an enhanced thermogenic effect of amino acids and fat in S also implies the need for more complete investigations of the impact of these substrates in modifying oxidative and energy metabolism in S.