The effects of starvation, glucocorticoid administration, and the feeding of an all-concentrate diet, on the 14CO2-fixing activities of pyruvate carboxylase (EC. 18.104.22.168), phosphopyruvate carboxylase (EC. 22.214.171.124) and propiony1-CoA carboxylase (EC. 126.96.36.199) were measured in liver from lactating cows. Starvation caused a 26% decrease in propiony1-CoA carboxylase activity, glucocorticoid administration a 35% decrease in phosphopyruvate carboxylase activity, and the all-concentrate diet 24 and 41% increases in the activities of phosphopyruvate carboxylase and propiony1-CoA carboxylase respectively.