A substitute for rat milk [Messer et al., 1969 (1)] has been evaluated as a nutrient source to artificially feed rat pups from 4 days after birth. The rat milk substitute has a normal fat concentration, suboptimal protein concentration and a high carbohydrate concentration when compared to natural rat milk. Rat pups artificially reared on the mild substitute by intermittent infusion via miniaturized intragastric cannulae have: 1) atypical ketone body metabolism: lower than normal concentration and turnover of D-(--)-3-hydroxybutyrate in blood and less than normal amounts of D-(--)-3-hydroxybutyrate used for respiration, 2) atypical carbohydrate metabolism: higher than normal insulin and galactose concentrations in blood and a greater than normal amount of glucose used for respiration, and 3) atypical amino acid levels: the concentrations of several amino acids in blood were 60% or less than normal, and the concentration of taurine in plasma was negligible. We observed frequent head tremors, hyperreactivity to handling and about a 20% incidence of cataracts in rat pups reared on the milk substitute. We conclude this rat milk substitute is not suitable as a nutrient source for the developing rat pup.