SummaryIn this paper, the 4 major pathogenetic mechanisms of acute renal failure, arc critically reviewed. It is highly probable that in the ischemic models, both tubular obstruction and tubular back leakage play a dominant role; on the other hand, in the nephrotoxic models, hemodynamic intrarenal changes as preglomerular vasoconstriction and/or changes in Kfare more important, especially in the early initiation phase of these models. The tubular lesions occurring in established ARF, lead to failure of urinary concentration and decrease in tubular sodium reabsorption and are reflected in changes in urinary composition.The urinary parameters, especially fractional excretion of sodium (FENa) and renal failure index (RFI) can be used in the differential diagnosis between prerenal and renal failure. However, in clinical ARF where tubular obstruction plays a major role, both the FENaand RFI can be low.