1. The activities of two hepatic enzymes that participate in the regulation of amino acid oxidation and urea synthesis were measured in lactating rats (day 15 of lactation) and virgin controls. The enzymes were alanine aminotransferase (EC 2.6.1.2) and argininosuccinate synthase (EC 6.3.4.5). Carcasses of the dams were also analysed. 2. Changes in the activities of both enzymes in dams fed ad lib. on a diet containing an excess of protein indicated that amino acid oxidation was depressed. In dams restricted in protein to the level of intake of their controls but allowed to satisfy their needs for energy, enzyme activities were significantly reduced. In these animals lean tissue catabolism supplemented the dietary protein supply. 3. This adjustment in protein metabolism which effectively spares protein for milk-protein synthesis could be explained either by a reduction in the availability of substrate in the liver, or by the intervention of an anabolic hormone secreted in lactation.