1. Heat production, calculated according to the respiratory quotient methods, HE(RQ), and the carbon nitrogen balance method, HE(CN), was compared using the results from a total of 326 balance trials with 56 castrated male pigs fed different dietary composition and variable feed levels during the live wt range 20-120 kg. 2. HE(RQ) was generally higher than HE(CN). 3. HE(RQ) depended on feed composition and feed level and, consequently, on the synthesis and deposition of depot fat. For RQ-values above 1, HE(RQ) was overestimated. 4. HE(CN) was generally underestimated, because N- and C-balances in general were overestimated due to losses in excreta. 5. The difference between HE(RQ) and HE(CN) was not constant. It was smallest when fat synthesis and fat deposition were low (2-3.5%) and greatest when fat synthesis and fat deposition were high (7-9%). The presence of dietary fatty acids may affect the difference. 6. In pigs receiving a cereal based diet, HE(RQ) may be expected to give 3-4% higher values than HE(CN), but in case easily available carbohydrates (glucose, sucrose) or high-fibre diets are provided, the differences may be larger. 7. Both methods were carried out with similar accuracy and precision. The results indicated that both methods are equally reliable.