Efficiency of protein utilization and lysine metabolism were studied in growing rats of the Zucker 13M strain, both obese and lean, and in the Charles River CD strain. When graded levels of wheat gluten or wheat gluten supplemented with lysine were fed to these three types of rats, no significant differences in the efficiency of protein utilization were found. However, under these dietary conditions, the Zucker obese rats appeared to be about 167% more efficient in energy utilization than the Zucker lean rats or the Charles River strain. The response of liver lysine-ketoglutarate reductase to the dietary treatment was similar in these three types of growing rats. The enzyme activity was induced by dietary lysine. Rats fed a lysine-deficient purified diet had a significantly lower enzyme activity than those fed a protein-free diet indicating that the same adaptive mechanism for lysine conservation as previously observed in adult rats also occurred in the growing animal. Interaction between dietary and genetic factors on fatty liver was found in this study. The most severe fatty liver was observed in the Zucker obese rats fed the most lysine-deficient diet containing 8% wheat gluten protein.