High, low, and zero quantities of methionine, histidine, or lysine were infused intravenously into lactating cows fed a corn–corn silage–urea ration to assess whether any of these amino acids were limiting for milk production. Feed intake was significantly higher (P < 0.05) with the low level of methionine infusion than with either the high level or saline infusions. Milk yield was not affected (P > 0.05) by any treatments, but protein yield was significantly greater (P < 0.05) with the low than with the high or zero levels of infusion of methionine, and the infusion of either level of histidine significantly decreased (P < 0.05) milk protein yield compared with zero level of infusion. The changes in levels of plasma amino acids that occurred during these studies have been interpreted as implying that methionine was the first limiting and either valine, leucine, or isoleucine was the second limiting amino acid for milk production.