Rat livers were perfused by the non-recirculating technique with medium containing [15N] glycine and sodium benzoate. At various times, the 15N-concentration and the isotopic enrichment of perfusate hippurate, albumin, globulines, urea, creatinine and other fractions were measured by MS. Hippurate was used to represent the intracellular precursor pool for albumin synthesis. After start of glycine administration the ratio APE product/APE precursor pool had reached approximately maximal values. After 30 min, the precursor pool size and its use in protein synthesis decreased due to amino-acid-deficient perfusion with no further decrease between 30 and 105 min. Additionally, rates of protein synthesis were determined. The albumin synthesis rate was 3.27 mg/g liver weight per hour.