SummaryNeonatal pigs were evaluated as a model for protein metabolism of infants. Milks were given in which the protein (15 or 30 g/liter) was supplied by casein and whey in the proportions of 80:20 or 40:60. Severe or moderate restriction of feed intake was also compared. Whey-predominant milks reduced the concentration of urea N in blood plasma and the proportion of urea N in urinary total N. These effects are consistent with most previously reported data from infants, particularly those of low birth weight. The efficiency of N retention was improved at the higher concentration of dietary protein. Whey-predominant milks also reduced the amount of N retained in the stomach. The magnitude of some of these effects was influenced by the level of feed intake. Total N retention reflected protein intake but was not affected by protein source. N retention determined by a balance procedure exceeded that calculated from carcass analysis, but the effect of protein intake was similar from either procedure.