Chronic renal failure (CRF) represents a world health problem. The aim of this paper is to evaluate the effect of ozone therapy, in the renal function and morphology, in an experimental model of CRF. Rats were divided into 4 groups: 1-negative control, rats without any treatment; 2-positive control, rats submitted to 5/6 reduction of the total renal mass (right kidney nephrectomy, with 2 branches of the renal arteria, of the left kidney, clamped); 3-ozone, as group 2, but receiving, rectally, 7 sessions of ozone (0.5 mg/kg), previously to the surgery procedure and twice per week (during 10 weeks) after the partial nephrectomy; 4- oxygen, as group 3, but receiving oxygen instead of ozone. Ozone oxidative preconditioning effect protected the tissues against the oxidative stress present in the CRF, being in correspondence with the positive histological results obtained