We evaluated the effects of chronic renal failure (CRF) on the sperm fertilizing potential of rats. CRF was induced in 20 male 8-week-old Wistar rats (group A) by performing 5/6 nephrectomy in two stages. An additional 10 rats underwent a two-stage sham operation and served as a control group (group B). Seven weeks after the second operation, serum urea and creatinine concentrations were evaluated. Caudal epididymal spermatozoa from rats with CRF but not from controls had been filtrated via Sperm Prep tubes to isolate the sperm fraction showing strong forward progressions. Spermatozoa were processed for insemination of ten mature oocytes. After 18 and 36 h of insemination, the percentage of oocytes with two pronuclei and cleaved oocytes were evaluated, respectively. Serum urea and creatinine concentrations were significantly higher in group A than in group B. There was no significant difference between groups A and B in the percent sperm motility in final suspensions. The percentage of oocytes with two pronuclei and cleaved oocytes were significantly lower in group A than in group B . The present study suggests that CRF has adverse effects on the overall sperm fertilizing capacity.