I . In order to establish whether or not there is a change in the relative rates of anabolism and catabolism of amino acids in the livers of lambs as they mature, a study has been made in isolated perfused livers obtained from foetal, suckling and ruminating lambs. The livers were perfused in a recirculating system and a mixture of amino acids (casein hydrolysate) was infused as substrate in the presence and absence of glucagon.2. The oxygen consumption of the livers per unit weight increased as the lambs matured but the uptake of a-amino nitrogen declined. A comparison of the rates of 0, consumption and substrate uptake indicated that in foetal lambs less than 20 of the amino acids could have been oxidized, whereas in ruminating lambs all the amino acids taken up could have been oxidized.3. In livers obtained from foetal lambs, the rate of urea production per unit weight of liver was approximately double that observed in ruminating lambs. Comparison of the rate of urea production and a-NH2-N uptake indicated that more than 60 % of the substrate uptake was used for non-catabolic processes in the foetal lambs, whereas in ruminating animals all the a-NH,-N taken up could be accounted for as urea.4. Gluconeogenesis could not be detected in lambs at any age studied and glucagon had no effect on any of the parameters studied.

The effect of glucagon on the rate of glucose production in the presence of alanine, of lactate and of propionate has been studied in isolated perfused livers obtained from foetal, suckling and weaned lambs.Glucose utilization occurred in foetal livers in the absence of glucagon whereas addition of glucagon resulted in a net production of glucose from the three substrates. In livers from suckling and weaned lambs addition of glucagon was not essential for glucose production to occur and the stimulation of gluconeogenesis by added glucagon was less evident in the older lambs. The effect of glucagon was not dependent on changes in the rate of substrate uptake.It is concluded that gluconeogenesis will be induced by glucagon in foetal lambs when their mothers are severely undernourished, and that placentally derived lactate could contribute up to 90 % of the glucose synthesized.