1. The relation between endogenous urea metabolism and albumin synthesis has been studied in ten patients with chronic renal failure and in four normal subjects, after single intravenous injections of [ 14 C]urea, [ 15 N]urea and 125 I-labelled albumin. 2. The rate of urea synthesis was determined from the dynamics of plasma [ 14 C]urea specific radioactivity and the rate of urea metabolism was estimated from the relative rates of urea synthesis and urea appearance in urine and body water. Deconvolution analysis of plasma [ 15 N]albumin enrichment and 125 I-labelled albumin radioactivity yielded the cumulative incorporation of 15 N into total exchangeable albumin and enabled calculation of the absolute rate of urea nitrogen utilization for albumin synthesis. 3. Although the mean absolute rate of urea degradation in uraemic patients (3.7 mmol/h) was higher than in normal subjects (2.3 mmol/h) there was no significant positive correlation between urea degradation and plasma urea concentration. 4. In uraemic subjects, there was a significant positive correlation between urea synthetic rate and urea degradation rate. 5. The rate of utilization of urea nitrogen for albumin synthesis was low, but was very much higher in uraemic subjects (mean 83.8 μmol/h) compared with normal subjects (mean 6.4 μmol/h), as was the provision by urea of the nitrogen required for albumin synthesis in uraemic subjects (2.37%) compared with normal subjects (0.13%). 6. The efficiency of utilization of urea nitrogen for albumin synthesis was higher in the uraemic patients (1.3%) than the normal subjects (0.2%), and was higher in those patients with chronic renal failure who received a 30 g protein diet than those on 70 g of protein. A significant negative correlation was noted between efficiency of urea nitrogen utilization and the rate of synthesis of albumin. 7. These studies suggest the presence of a mechanism for the conservation of urea nitrogen in chronic renal failure which is unrelated to the extent of urea degradation, and which can only be partly explained by the higher proportion of intraluminal gut nitrogen derived from urea.