The energy value of NSP has been expressed as their metabolizable energy (ME) content. The aim of the present study was to determine whether differences in ME and net energy (NE) contents were similar for insoluble and soluble NSP. Nine healthy young men were offered three diets according to a Latin-square design (3 x 3) with three repetitions: diet C (control), diet B (control + 50 g sugarbeet fibre/d) and diet I (control + 50 g commercial inulin/d). After a 16 d adaptation period to NSP isolate, food intake was controlled (duplicate meal method) and faeces and urine were collected for 8 d. A period of 60 h was devoted to measurement of energy expenditure (EE) by whole-body indirect calorimetry. NSP-isolate ingestion induced significant increases in the number of defecations and stool weight resulting from increases in water, DM and microbial mass excretion. After deduction of microbial N, differences in faecal N excretion between diets were not significantly different. Urinary N excretion was slightly decreased by sugarbeet fibre or commercial inulin ingestion but the N balances for the diets were not significantly different. Diet energy, N and lipid apparent digestibilities decreased by only 1-2%. Commercial inulin was entirely fermented and fermentability of sugarbeet fibre averaged 0.886 (SD 0.117). Sugarbeet fibre and commercial inulin ME values averaged 10.7 (SD 1.2) and 13.0 (SD 2.3) kJ/g DM respectively. NSP-isolate ingestion caused significant (sugarbeet) and nonsignificant (inulin) increases in daily EE. The maintenance NE contents of sugarbeet fibre and inulin averaged 5.0 (SD 5.0) and 11.9 (SD 1.3) kJ/g DM respectively. Differences in maintenance NE contents of NSP isolates were much greater than differences in ME values.