**AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** US DAIRY FARMERS MAKE SIGNIFICANT INVESTMENTS TO FEED FAT SUPPLEMENTS TO THEIR CATTLE IN ATTEMPTS TO INCREASE DIETARY ENERGY DENSITY, MILK FAT PRODUCTION, AND HERD PROFITABILITY. HOWEVER, THERE ARE MANY GAPS IN OUR KNOWLEDGE OF FATTY ACID (FA) METABOLISM AND THE EFFECT OF DIETARY FA ON DE NOVO FA SYNTHESIS IN THE MAMMARY GLAND. IT IS OUR UNDERSTANDING THAT ONLY ONE STUDY HAS DIRECTLY INVESTIGATED THE EFFECT OF INDIVIDUAL FA SUPPLEMENTATION ON MAMMARY FA UPTAKE. THE MAMMARY ARTERIOVENOUS DIFFERENCE APPROACH HAS BEEN UTILIZED EXTENSIVELY TO DETERMINE INDIVIDUAL AMINO ACID REQUIREMENTS IN THE LACTATING DAIRY COW AND IS EXPECTED TO BE A VALUABLE TOOL TO INVESTIGATE KEY QUESTIONS REGARDING THE UPTAKE EFFICIENCY OF INDIVIDUAL FA. WE AIM TO USE THIS ARTERIOVENOUS APPROACH TO CHARACTERIZE THE EFFECT OF PALMITIC AND STEARIC ACID ON MAMMARY FA UPTAKE AND DE NOVO FA SYNTHESIS USING BOTH DOSE-ESCALATION AND LONGER-TERM INFUSION MODELS. WE WILL ALSO ANALYZE THE EFFECT OF LONGER-TERM FA INFUSION ONMAMMARY EXPRESSION OF FA METABOLISM GENES. WE HYPOTHESIZE THAT THE RATE AND MAXIMUM CAPACITY OF MAMMARY FA UPTAKE WILL DIFFER BETWEEN PALMITIC AND STEARIC ACID, LIKELY DUE TO BOTH DIFFERENCES IN ENZYME AFFINITY AND THEIR MODIFICATION OF GENE EXPRESSION ASSOCIATED WITH FA METABOLISM IN THE MAMMARY GLAND. THIS RESEARCH WILL PROVIDE NOVEL INSIGHT REGARDING THE METABOLIC ADAPTATIONS ASSOCIATED WITH INCREASING INDIVIDUAL FA. IT WILL ALSO HELP INFORM DAIRY FARMERS, NUTRITIONISTS, AND FEED COMPANIES HOW TO BETTER UTILIZE FAT SUPPLEMENTATION IN LACTATING COW RATIONS TO INCREASE MILK FAT PRODUCTION AND MAXIMIZE RETURN ON INVESTMENT.