1. The effect of uraemia on the rates of protein synthesis and protein degradation in liver, heart and vastus lateralis muscle were examined in the rat. Uraemia was induced by a five-sixths nephrectomy and the rates of protein turnover were compared with pair-fed sham-operated littermate controls. 2. The procedure produced plasma concentrations (means +/- SEM) of urea of 7.3 +/- 0.4 mmol/l in control and 39 +/- 2 mmol/l in uraemic rats, and of creatinine of 41 +/- 1 mumol/l in control and 133 +/- 7 mumol/l in uraemic rats. 3. Uraemia reduced the rates of protein synthesis in liver, heart and muscle by 35 +/- 5, 4.0 +/- 1.2 and 4.0 +/- 0.6%/day, respectively, compared with control rats (P less than 0.01). Since the reductions in tissue growth rate were too small to be accounted for by the reduction seen in protein synthesis alone, this implied that protein degradation was also reduced. Uraemia also caused an 18% reduction in the rate of growth as measured by the increase in tail length (P less than 0.01). 4. Uraemia reduced both protein synthesis and protein degradation. Protein synthesis exceeded protein degradation in all tissues. This would cause a reduced rate of protein accumulation in the uraemic compared with the control rats, and hence a reduced rate of growth.