1. Four cows in early lactation were given continuous infusion into the abomasum of 0, 200, 400 or 600 g lactic casein/d according to a Latin-square design. Each period was of 14 d and the Latin square was followed by 7 d in which an infusion of 800 g glucose/d was given. The basal diet was given at a level which provided sufficient nitrogen and energy for 10 kg milk yield/d. 2. Infusion of casein resulted in significant increases in milk yield, milk N yield and milk energy yield; milk N increased progressively but milk energy reached a maximum at 400 g casein/d. Milk yields and composition when glucose was infused resembled those seen on the zero casein treatment. 3. N-balance measurements indicated a severe deficit (-20 g/d) on the zero casein treatment and a progressive increase to +7 g/d as casein increased; N equilibrium was achieved at about 400 g casein/d. The regression of net productive N on N intake (P less than 0.001) indicated that the efficiency of utilization of dietary N did not differ between treatments. 4. Heat production increased with increase in casein infused (P less than 0.05) but remained a constant proportion of the metabolizable energy (ME) intake. Energy balances were negative and did not differ significantly between treatments but calculation of the protein and fat components indicated a threefold increase in body fat mobilization in response to the first increment of casein. Milk yield adjusted to zero energy balance was significantly related to ME intake (P less than 0.001) but the efficiency of energy utilization was not affected by the level of casein infusion. 5. The concentrations of glucose, beta-hydroxybutyrate and non-esterified fatty acids in plasma did not differ between treatments but the concentration of urea in plasma increased markedly (P less than 0.05) at the highest level of casein addition. Insulin concentrations increased and growth hormone decreased (both P less than 0.05) with increase in casein infusion. 6. The concentration of total amino acids (AA) in plasma increased up to 400 g casein/d and then declined. Changes in concentration and in the ratio of essential:total AA indicated a very high extraction rate of essential AA at the lower levels of casein infusion. 7. The observed lactational responses are discussed in relation to the ratio of protein:energy in the absorbed nutrients.(ABSTRACT TRUNCATED AT 400 WORDS)