The composition and intake of milk by mother-reared puppies was studied to compare protein and energy intakes of puppies with estimated requirements. Milk samples were obtained from five beagle bitches over the period of 7-37 days postpartum. Dog milk contained on average 22.7% dry matter, 9.47% fat, 7.53% protein, 3.81% sugar and 146 kcal gross energy per 100 g. Protein comprised 31% of milk energy. Nonprotein nitrogen averaged 0.054%, equivalent to 4.4% of total nitrogen. Milk intakes of puppies in the five litters were estimated from water kinetics following administration of deuterium oxide (D2O). D2O dilution indicated that body water comprised 72-73% of puppy body weight, and fractional turnover rate of body water averaged 0.15-0.17% per day in weeks 3 and 4 postpartum. Milk intakes were calculated as 160 +/- 5.4 g (mean +/- SEM) at 19 days and 175 +/- 5.3 g at 26 days, equivalent to 17.0 and 14.6% of body weight, respectively. Daily milk yields of the bitches averaged 964 g at 19 days and 1054 g at 26 days. Dry matter intakes of the puppies were equivalent to 3.9 and 3.3% of body weight at 19 and 26 days, respectively. Gross energy intakes averaged 223-224 kcal/ kg0 .75 per day, and protein intakes averaged 0.33-0.36 g per gram body weight gain at these ages. Estimates of the energy requirements of young puppies by the National Research Council appear to be too high.