Ten cows were used in 4 x 4 Latin squares with an extra period for estimation of carry-over effects to study plasma amino acid responses to jugular infusion of amino acids at early, mid, and late lactation. Methionine (M), methionine + lysine (ML), and methionine + lysine + phenylalanine (MLP) in a balance with glutamic acid (C) were infused at 25% of the amino acid content of pretreatment milk via the jugular vein. Rations were formulated at 16, 15, and 14% crude protein and 15, 18, and 21% crude fiber for 30, 120, and 240 day lactational groups. Carry-over effects were present only in arterial blood at 240 days. Differences in milk production were not significant, although production increased on M, ML, and MLP at 30 days. Plasma amino acid responses of nonruminants were utilized to aid in evaluation. Arterial and venous plasma amino acid concentrations were lowest on ML at 30 days, while amino acid uptake was significantly increased by M. At 120 days plasma responses were inconclusive and non-significant. The amino acid uptake trend followed that observed at 30 days. At 240 days plasma amino acid concentrations were lowest with ML, while uptake was increased on MLP. Essential amino acids were ranked in potential orders of limitation by amino acid extraction and utilization for milk protein by the mammary gland. Order of limitation differed between calculation methods but was similar for all three stage.s of lactation. Orders of limitation were as follows: Amino acid extraction - methionine~ lysine, arginine, leucine, phenylalanine, isoleucine, threonine, valine, and histidine. Amino acid utilization - phenylalanine, threonine or histidine, valine, leucine, methionine, lysine, isoleucine, and arginine. Since there was no substantial change in order of limitation with infusion, either protein intakes were above requirements or amino acids not infused are candidates for limitation. However, among amino acids infused, the parameters observed suggested lysine or methionine at 30 days, methionine at 120 days, and no apparent choice at 240 days. / Master of Science