1. Procedures are described to prepare nutritionally adequate rat milk-substitutes by modifying commercially available processed cow's milk, rich in carbohydrate and low in protein and fat compared with rat's milk. 2. Premilk formulas, prepared as intermediates in the preparation of rat milk-substitutes, are rich in protein but low in their concentration of fat, carbohydrate, and minerals when compared with rat's milk. 3. Premilks were supplemented with lactose, vitamins, minerals, fat as oil mixtures, certain amino acids and other constituents to yield rat milk-substitutes which resemble the known composition of rat's milk in their properties and composition. 4. Detailed analyses of the milk-substitutes show them to be comparable to rat's milk in energy content, pH, osmolarity, the concentration of the macronutrients, fat, protein and carbohydrate, and the major minerals. 5. Rat pups were artificially reared from postnatal day 4 or 5 until days 16-18 by fitting them with gastric cannulas through which the milk-substitutes could be infused automatically. 6. The nutritional impact of the milk-substitutes was assessed by a comparison of growth and metabolic characteristics for artificially reared rats with age-matched sucking rats reared by their mother. 7. Indices which were taken to be appropriate included (a) body-weight gain; (b) the concentration in blood of protein, amino acids, ketone bodies, carnitine, glucose, galactose, lactate, insulin, and the electrolytes calcium, sodium, potassium and chloride; (c) the turnover of glucose and 3-hydroxybutyrate; (d) the concentration in brain of protein, cholesterol, cerebroside sulphate and the activities of the enzymes pyruvate dehydrogenase (EC 1.2.4.1), 3-oxo-acid-CoA transferase (EC 2.8.3.5) and acetoacetyl-CoA ligase (EC 6.2.1.16). 8. The studies suggest that milk-substitutes approximating to rat's milk in composition promote acceptable metabolism in the artificially reared rat pup.