In recent years some approaches were developed to estimate the parameters of transport of amino acids and other substrates into the cell in vivo using arteriovenous concentration difference and volume blood flow data with their recalculation according to kinetic model of the system: capillary - interstitium - cell. With the use of literature data and the own results the authors analysed the present feasibility of this method in the studies of mammary metabolism in productive ruminants The model estimates were compared with independent measurements (with ¹³С-amino acids), the verifying experiments were conducted and some unresolved problems and errors of interpretation were investigated. The method is recommended to study the adaptive shifts in the activity of amino acid transport into the mammary secretory cells during milk formation in various nutritional states.