In experiments on surgically modified ruminants were obtained data determining quantitatively conversion of feed components in the rumen and intestine and subsequently incorporation of conversion products into metabolic pool and utilization of substrates for biosynthesis of milk component and heat production. Parameters of transport, absorption and utilization of the substrates of carbohydrate, lipids and amino acids in the intestinal wall and mammary gland at different phases of lactation and feeding condition were defined. Factors influencing the ratio of substrate of energy metabolism to: feeding level, proportion of substrate absorption, lactation stadium. Conducted experiments indicate a necessity of future studies on protein and lipid metabolism for improvement of feeding standards for dairy cows.