A diet containing, on a dry matter basis, 41.8% acid detergent fiber and 17.9% crude protein, of which 55% was obtained from urea, was compared for lactating dairy cows with a diet in which nitrogen was supplied from vegetable protein, and most of the total digestible nutrients were from starch. Dry matter intake was 17.6 and 14.9 kg per day (P < 0.05), 4% fat-corrected milk production was 23.2 and 20.8 kg per day (P < 0.05), and total levels of essential amino acids were 159.78 and 176.14 μg/ml (P < 0.05), respectively, for the cows receiving the protein–starch diet, compared with those receiving the urea–fiber diet. Except for serine, proline, cystine, asparagine and threonine, the plasma amino acids were lower in cows fed the protein–starch diet than in cows fed the fiber–urea diet. The plasma content of all amino acids except aspartic acid, glutamic acid and cystine increased consistently during the 1st mo of lactation. This elevation was coincidental with an increasing daily rate of feed consumption and a more rapidly increasing milk production.