The objective was to determine the effects of excess MP supplementation from feedstuffs differing in rumen degradability of protein on plasma AA concentrations. As part of a larger study, we hypothesized excess MP supplementation from feedstuffs differing in rumen degradability would alter reproductive performance of beef cows around the time of ovulation, potentially due to changing circulating AA concentrations. Non-pregnant, nonlactating mature beef cows (n = 18) were allotted by age, BW and BCS to ad libitum corn stalks and 1 of 2 isocaloric, isonitrogenous dietary supplements consisting primarily of either (1) corn gluten meal (GLM) or (2) soybean meal (SBM), supplemented at 150% of NRC MP requirements. Cows were individually supplemented once daily for 58 d and coccygeal blood samples were taken on d 47 for AA analysis. Data were analyzed using the MIXED procedures of SAS. On d 47, total AA, essential AA, nonessential AA, glycogenic AA and branched-chain AA were not different (P ≥ 0.21) between treatments. However, total ketogenic AA tended to be greater (P = 0.07) in excess GLM than excess SBM supplemented cows. When expressed as a percent of total AA, arginine, lysine, threonine, tryptophan and valine were lesser (P ≤ 0.05) in GLM than SBM supplemented cows. However as a percent of total AA, leucine, methionine and phenylalanine were greater (P ≤ 0.03) in GLM than SBM cows. In addition, as a percent of total AA asparagine and proline were greater (P < 0.05) in cows supplemented with GLM than SBM. As a percent of essential AA, arginine, lysine, valine, tryptophan and threonine were lesser (P ≤ 0.04), and methionine, phenylalanine and leucine were greater (P ≤ 0.03) in GLM than SBM. These data illustrate that supplementing MP at 150% of NRC requirements from feedstuffs differing in rumen degradability may change the profile of circulating plasma AA around the time of ovulation in beef cows consuming low quality forage. However, the effects of this change in AA profile on reproductive function of beef cows are still largely undetermined and warrant further investigation.