1. Complete 24 h energy and nitrogen balances were measured for eight subjects both while consuming a basal diet supplemented with 49 g saccharose/d (diet S) and while consuming the same basal diet but supplemented with 50 g lactitol monohydrate/d (diet L).2. The subjects ate the two diets for 8 d. Faeces and urine were collected for the final 4 d. Exchange of respiratory gases (oxygen, carbon dioxide, hydrogen and methane) was measured during the final 72 h while the subjects stayed in an open-circuit respiration chamber, 11 m³, and simulated office work. Before eating diet L, subjects ate 50 g lactitol daily for 10 d.3. On diets L and S, faecal moisture content averaged 0.787 and 0.753 g/g respectively, the difference being significant (P < 0.05). On diet L, energy and nitrogen digestibilities and energy metabolizability averaged 0, 922, 0.836 and 0-881 respectively, and on diet S 0.935, 0.869 and 0.896 respectively; the differences were also significant (P < 0.05). Urinary energy losses and N balances were not significantly different for the two diets.4. In all subjects only traces of methane were produced but hydrogen production differed significantly (P < 0.05) for diets L and S, being 2.3 and 0.4 litres (normal temperature and pressure)/d respectively.5. Intakes of metabolizable energy (ME) were corrected, within subjects, to energy equilibrium and equal metabolic body-weight. The corrected ME intakes did not show differences between diets. However, when on diet L the subjects were probably less active than when on diet S because differences within subjects of ankle actometer counts between diets showed a high correlation with the corresponding differences in corrected ME intakes (r 0.92). Further correction of ME intake toward equal actometer activity showed a significant (P < 0.05) difference between diets: for maintaining energy equilibrium 5.6 (SE 0.8; P < 0.05) % more ME from diet L was needed than from diet S. The reliability of this 5.6% difference depends on whether or not one ankle actometer gives an accurate picture of the subject's physical activity.6. The energy contribution to the body is clearly smaller from lactitol than from saccharose, certainly due to the effect of lactitol on digestion, and probably also due to the effect on the utilization of ME.