Despite a nutritional approach splitting protein and energy inputs in the models used to balance dairy rations, many studies indicated important interactions between energy and protein metabolism. For example, in the dairy cow, increases in milk protein and lactose yields are both induced with increased protein supply. This increase in lactose yield is not always paralleled by an incremental increase in mammary glucose uptake but is associated with an increased uptake of branched-chain amino acids (AA). Although AA are used for gluconeogenesis, Bequette et al. (2005) demonstrated in vitro that some essential AA could contribute to lactose synthesis by mammary cells supporting earlier observations that arterial glucose would not be the only precursor of milk lactose (Bickerstaffe et al., 1974). Therefore, the objective of this work was to study the interactions between AA and glucose metabolism in dairy cows receiving different AA mixtures and to determine if AA could contribute to lactose synthesis within the mammary gland.