Objectives: To determine the histological effects of sildenafil administered as a preconditioning drug before a period of warm ischemia, in order to protect kidneys in cases of renal ischemic surgical procedures.-Material and methods: Group 1 (n=10): Control with 15min, of warm ischemia during clamping of the vascular renal pedicle. Biopsies will be done from it before, after 15 min, and 24 hours after the procedure for pathological assessment. Group 2 (n=10) Control with 30 min of warm ischemia (done as in group 1).Renal biopsies done before,after30 min, and 24 hours after the procedure for pathological assessment of warm ischemia and reperfusion. Group 3 ( n=20)Oral dose of 1.5 mg/kg (average 100mg) sildenafil 1 hour before surgery and15 min, of warm ischemia as in group 1.Biopsies from kidney before ischemia,after15 min and 24 hours after the procedure are carried out for pathological assessment of warm ischemia after sildenafil intake. Group 4 ( n=20) Oral dose of 1.5 mg/kg (average 100mg) sildenafil 1 hour before surgery and 30min, of warm ischemia as in group 2.Biopsies from kidney before ischemia, after 30 min and 24 hours after the procedure are carried out for pathological assessment of warm ischemia after sildenafil intake. A conventional and electron microscopic study was made in all cases, after the procedure.-Results and Conclusions: Our results suggest a possible protective effect of sildenafil as a preconditioning drug that can be safely used warm ischemic kidney, without toxic effects.