1. Four mature Friesian cows were used in a Latin square design experiment to measure the effects of abomasal casein infusion on milk production and the kinetics of glucose, acetate and palmitate in jugular venous blood.2. The basal ration contained 11 0 g crude protein/kg dry matter and was offered at the rate of 4 kg hay, 3 kg alkali-treated straw cubes plus 9 kg concentrates/d. Treatments were infusion of casein into the abomasum at the rate of 0, 240 or 460 g/d. The experiment started 1 week post partum. Experimental periods lasted 21 d.3. [6-SH]glucose, [9, 10-3H]palmitate and [U-"C]acetate were continuously infused into one jugular vein for measurements of whole-body flux rates.4. The yield of milk protein was increased significantly (P < 0.05) by the first increment of casein infusion, with no further increases.5. Casein infusion had no effect on blood concentrations of glucose, acetate, palmitate or growth hormone.Mean flux rate of glucose increased with each increment of casein. The effect at 460 g/d was higher (P < 0.1) than with no infusion. Flux rates of palmitate and acetate were increased (P < 0.1) by the first increment of casein.The proportion of blood bicarbonate derived from acetate increased (P -= 0.05) with the first increment of casein.6. Protein status of cows early in lactation influences the dynamics of glucose and fatty acid metabolism.