Milk intake and energy expenditure was measured by doubly-labelled water (DLW, ²H₂¹⁸O) in reindeer calves during their first weeks of life. Two successive experiments (each 7 days) were conducted in four calves at the ages of 1–2 and 3–4 weeks. The calves were freely suckling their mothers which were fed high-protein concentrates. The milk intake of the calves was on average 1.28 kg/day (1.04–1.50 kg/day) during the first experiment and 1.47 kg/day (1.23–1.88 kg/day) during the second experiment. The energy expenditure of the calves was on average 5.87 MJ/day at the age of 1–2 weeks and 7.42 MJ/day at the age of 3–4 weeks. The milk output of the mothers, measured by a milking machine, was on average 1.37 kg/day (0.88–1.80 kg/day) at the end of the first experiment and 1.12 kg/day (0.52–1.74 kg/day) at the end of the second experiment. Milk contained on average 10.8 % fat, 8.2 % protein, 4.5 % lactose, 23.7 % dry matter and 6.9 kJ/g gross energy. The body weight gain of the calves correlated with the milk intake and energy expenditure. The milk intake correlated also with the body weight and feed intake of the mothers. The results show that DLW method suits for the measurement of milk intake of reindeer calves during the peak suckling period when their major water source is milk. The benefit of the method is that the lactation remains undisturbed and the energy expenditure can be measured simultaneously. The disadvantage is that the method is technically demanding and expensive.