The aim of this study was to evaluate whether the poly(adenosine diphosphate[ADP] ribose) polymerase (PARP) pathway is activated by experimental left varicocele. Rats underwent partial ligation of the left renal vein to induce experimental varicocele, and left testes were analyzed 13 weeks after surgery. Tubule degeneration was evaluated by Johnsen score. Expression of 4-hydroxy-2-nonenal (4-HNE) modified proteins, PARP-1, and poly(-ADP-ribose) (PAR) was detected by immunohistochemistry and Western blot. The degree of apoptosis within testes was determined by terminal deoxynucleotidyl transferase mediated deoxyuridine triphosphate nick end labeling. Light microscopy revealed testicular damage comprising various degrees of seminiferous tubule degeneration. Germ cell apoptotic index and 4-HNE, PAR, and PARP-1 expression in germ cells increased after varicocele induction. Increased oxidative stress and PARP overactivation in testes might be important with regard to impaired testicular function associated with varicocele.