Background: Magnesium and phosphorus are one of the electrolytes whose balance is altered during advanced chronic renal failure (CRD-V). It has already been reported in the literature that there is a progressive increase in urinary electrolyte excretion in CRD-V in relation with the glomerular filtration rate reduction, but it has not been extensively studied yet for these divalent ions. Therefore we decided to perform a study for evaluating compared values of urinary and peritoneal fractional excretion of these divalent ions (magnesium and phosphorus) among patients on peritoneal dialysis with residual diuresis (CRD-V), stage III - chronic renal failure, and healthy volunteers. Results: Urinary fractional excretion (FEu) of magnesium (Mg) and phosphorus (P) were significantly (p= < 0.05) lower in healthy volunteers (FEuMg: 3 ± 1%, FEuP: 9 ± 0.5%), and significantly higher in CRD-V (FEuMg: 30 ± 4%, FEuP: 40 ± 0.2%). Regarding peritoneal fractional excretion (FEp) of magnesium and phosphorus, they were significantly higher (p= < 0.05) (FEpMg: 36 ± 5%, FEpP: 62 ± 9%) than the urinary ones in the same population: CRD-V. Conclusion: Magnesium and phosphorus urinary fractional excretion values were significantly higher in the CRD-V group, while these values were significantly lower than the peritoneal ones in the same population: CRD-V.