This thesis, presented in four sections, is concerned with the regulation of protein metabolism in the liver and small intestine of the rat. Section 1. A method was developed which allows for the measurement of rates of protein synthesis in tissues such as liver and intestine which synthesise and degrade protein very rapidly. The method employed a flooding amount of (14C)leucine (100umoles per 100g body weight) injected intravenously with measurement of the rate of incorporation of labelled leucine into protein over the following 10 minutes. Several experiments were performed to show that protein synthesis, per se, was unaltered by the flooding amount of leucine. Section 2. Employing the method described in Section 1, protein synthesis was measured in the liver and the individual components of the gastro-intestinal tract and in the whole animal. These measurements demonstrated the importance of these rapidly turning over tissues to overall protein metabolism in the whole animal. Section 3. Nutritional and hormonal perturbations leading to the loss of body protein were investigated. Rates of protein synthesis in the liver and small intestine have been examined under three conditions; namely, starvation, dietary protein-deprivation and diabetes. Each of these conditions caused a loss of protein from the liver and a lowered rate of synthesis but by different Intracellular mechanisms. In the intestine the synthesis of protein was depressed by starvation and protein-deprivation but maintained in diabetes. Section 4. The role of leucine as a regulator of protein synthesis was examined. Protein synthesis was measured using a flooding amount of (-H)phenylalanine in the presence and absence of l00umoles of unlabelled leucine. The impact of leucine on protein synthesis in liver, intestine and muscle of fed, starved and protein-deprived animals appeared to be negligible in vivo, inspite of many reports that leucine stimulated protein synthesis in vitro.