The aim of the study was to assess thermogenesis during parenteral feeding (group A) and after an oral test meal of a polymeric diet (group B). Carbohydrates, fat and protein ratios in the oral meal and parenteral mixture were: 51:34:15 and 76:11:13, resp. In both situations 20% of resting energy expenditure (REE) was administered-as a bolus oral meal or during 120 min. of intravenous infusion. Mean results of respiratory gas exchange of 15 min. periods were used for calculations. Basal respiratory quotient (RQ) in group A and B was 0.841 and 0.806 resp (difference not significant; NS); peak RQ was 0.910 and 0.924 resp. (NS). Peak RQ in group B was significantly higher than basal (p < 0.006). REE in group A and B was 1,416 +/- 0,231 kj/kg/15 min. and 1,322 +/- 0,226 kj/kg/15 min. resp. (NS). Peak thermic effect (expressed as the rise of REE in percent of the energy content of the nutrients) was: 4.778% and 5.135% resp. (NS). Postprandial thermogenesis is not dependent on the route of administration of nutrients (parenteral or oral). The proportion of substrate utilisation depends on the content of the meal or parenteral mixture.