Changes in the kinetics of leucine in the mammary gland were examined in four lactating goats (25, 38, 45, and 135 DIM) that were given an i.v. infusion of a mixture of 18 AA, not including leucine, to alter the availability of leucine to the gland relative to other AA. Arteriovenous monitoring of [1-13C]leucine kinetics across one-half of the mammary gland was conducted on the last day (d 6 or 7) of the saline (control) and the AA infusion periods. Although blood flow to the mammary gland and the arterial concentration of most AA other than leucine were increased by the AA infusion, milk and protein yields did not change. For goats in early lactation (n = 3), arterial leucine concentrations fell considerably during AA infusion; however, the arteriovenous difference of leucine was maintained, resulting in uncommonly low leucine concentrations in venous plasma (8 microM). Whole body leucine flux (protein synthesis plus oxidation) was unaffected by AA infusion, but, because whole body leucine oxidation was reduced, whole body utilization of leucine for protein synthesis increased. The AA infusion reduced mammary oxidation of leucine to approximately one-third of control values. These results suggest that leucine oxidation can be reduced considerably without affecting milk protein output; thus, leucine oxidation may not be an irrevocable consequence of mammary metabolism. If catabolism of other AA either by the gland or in the whole body can be reduced, then the efficiency of milk yield can be improved.