Estimates of total daily energy expenditure (TDEE) by heart-rate (HR) monitoring were compared with those made by the doubly labeled water (DLW) method in nine exclusively breast-feeding women. Subjects recorded HR and dietary intake daily during the 8-d, isotope-measurement period. Milk energy output was determined by 3-d test weighing and analysis of 24-h milk samples. Total energy output (milk energy and TDEE) averaged 12.36 +/- 1.03 MJ/d with DLW compared with 11.74 +/- 1.3 MJ/d with HR monitoring, a 5.8% difference (NS). Individual differences ranged from -27.1% to +17.6%. The high water turnover and relatively low level of activity during lactation made the slopes of deuterium and 18O disappearance more similar, resulting in increased error in estimates of TDEE by DLW. Therefore, the DLW method may not be appropriate for use in lactating women. There are considerable individual deviations in estimating TDEE by HR monitoring, but it is satisfactory for estimating TDEE of groups.