Biochemical changes in the blood following induction of renal failure by glycerol or mercuric chloride have been studied in 16 rats. Plasma creatinine, urea and Pi levels indicated that renal impairment followed the same time course in both renal failure models, with the severest effects on day 3 and returning to normal by day 7. Erythrocyte ATP and guanine triphosphate (GTP) levels were significantly elevated above control values on day 1 and remained elevated in both models. ATP/ADP and GTP/GDP ratios also increased in both models. In renal failure the increased purine ‘salvage’ in the erythrocyte may be attributed to accumulation of purine metabolites in the serum associated with increased P-ribose-PP levels due to elevated cellular Pi. Nucleotide changes in both these models are analogous to those found in chronic renal failure in man.