Total energy expenditures and intakes were simultaneously assessed in 18 free-ranging 284= lactating women (10 months postpartum) and compared to six similarly-sized, nonlactating, nonpregnant but multiparous women living in the same rural villages in the Guatemalan highlands. Energy intakes were estimated by the 24-hr recall method for each of 4 consecutive days. Energy expenditures were determined for 2 days by monitoring heart rate throughout the day and relating heart rate to oxygen consumption by individually-determined regression lines. The mean energy intake for the 4 consecutive days. Energy expenditures were determined for 2 days by monitoring heart rate throughout the day and relating heart rate to oxygen consumption by individually-determined regression lines. The mean energy intake for the 4 consecutive days was estimated to be 1929 ± 360 kcal/day (39.2 kcal/kg per day) for the lactating group; and 1876 ± 404 kcal/day (38.3 kcal/kg per day) for the nonlactating group. The 2-day mean energy expenditures were estimated to be 2007 ± 292 kcal/day for the lactating women (41.8 kcal/kg per day) and 1966 ± 382 kcal/day for the nonlactating women (40.1 kcal/kg per day). The way of life of both groups was judged 'moderately active' by 1973 FAO/WHO classifications. Most of the lactating women had been losing weight progressively during the past 6 months. Over the 10-week period prior to our measurements, the mean weight loss was more than 10 times greater in the lactating group (369 g/month) (P<0.01) than in the nonlactating group (-35 g/month) (ns). The high correlation (r=0.87) between weight loss and the reduction in the sum of the three skinfolds suggested adipose tissue loss. There were no significant differences between the two groups in terms of daily energy intake, daily energy expenditure, the energy cost of specific activities, or the pattern of activities throughout the day. The slope of the heart rate/oxygen consumption regression suggest adequate cardiorespiratory fitness. This study suggests that the energy cost of lactation was met to a greater extent by fat loss than by either increased energy intake, reduced energy expenditure, or both.