Isolated hindquarters of bilaterally nephrectomized and sham-operated rats were perfused in the presence and absence of 14C-labelled serine, respectively. After a perfusion period of 30 min 14C-serine was 4,074 ± 270 dpm/ml in the perfusion medium of sham-operated animals and decreased to 2,800 ± 190 dpm/ml in the medium of acutely uraemic rats. Muscle glycogen concentration in sham-operated animals was 1.10 ± 0.04 mg/g wet weight in the absence and 1.03 ± 0.11 mg/g in the presence of serine. In contrast, in acutely uraemic rats there was a glycogen concentration of 0.57 ± 0.09 mg/g in the absence of serine. Glycogen was increased in the presence of serine in the perfusion medium, the value being 1.50 ± 0.13 mg glycogen/g wet weight. Incorporation of labelled serine into skeletal muscle glycogen was significantly higher in acutely uraemic animals (15 ± 0.5 µmol/g glycogen) than in sham-operated animals (10 ± 0.4 µmol/g). The results are compatible with the hypothesis that serine increases muscle glycogen synthesis in acute uraemia.