Four multiparous Holstein cows were used in a 4 x 4 Latin square experiment with a 2x2 factorial arrangement of treatments. The objective of the experiment was to evaluate the effects of supplemental ruminally undegradable protein (RUP) on milk production, milk protein percentage and yield of cows receiving added fat in the form of whole cottonseeds (WCS). An evaluation of amino acid (AA) uptake by the mammary gland was also made. Treatments were 1) control, 2) WCS, 3) RUP, 4) WCS + RUP. The addition of WCS decreased milk protein production Supplemental RUP did not reverse the milk protein depression but the total effects may have been masked due to differences in CP intake.