[15N]glycine (95+%) was infused into 170- to 220-g rats at a constant rate of 2-8 mg [15N]glycine/h for 2-24 h. Two sets of experiments were done. In one set, the rats were killed at varying time intervals, the liver was removed, and the fractional rate of liver protein synthesis was estimated from the amount of 15N incorporated into liver protein, the enrichment of the liver tissue free amino nitrogen, and the time course. In the second set of experiments, the rats were killed after a [15N]glycine infusion of 18-22 h. The whole-body protein synthesis rate was estimated from the urinary 15N enrichment at plateau by the method of Picou and Taylor-Roberts (Clin. Sci. 36: 288-296, 1967). It was compared against the value found by measuring the 15N enrichment of the whole-rat homogenate and calculating the synthesis rate from the formula of Garlick et al. [Biochem. J. 136: 935-945, 1973). The results are i) The 15N enrichment of glycine in either liver protein or liver tissue free amino acids was proportional to the 15N enrichment of the mixed protein or tissue free amino acids, respectively. ii) Continuous infusion-isotopic plateau methods underestimate the fractional protein synthesis rate of rat liver. iii) The methods of Picou and Taylor-Roberts and of Garlick et al. gave similar values for the whole-body protein synthesis rate.