OBJECTIVE:To investigate histological changes in the contralateral testis of rats with unilateral testicular torsion and the protective effects of nitric oxide (NO) on possible damage. MATERIAL AND METHODS:Twenty-eight prepubertal male Sprague-Dawley rats were divided into four equal groups. Group 1 underwent a sham operation of the right testis under general anaesthesia. Group 2 underwent a similar operation but the right testis was rotated 720 degrees clockwise for 6 h, maintained by fixing the testis to the scrotum, and saline infused during the procedure. Group 3 underwent similar torsion but L-arginine methyl ester (a precursor of NO) was infused during the procedure. In Group 4, NG-nitro-L-arginine-methyl ester, a NO synthase inhibitor, was infused separately during the administration of L-arginine methyl ester and torsion. All the left (untwisted) testes were removed from rats 21 days after surgery and evaluated histologically, assessing seminiferous tubule diameter, loss of sperm and spermatids, loss of germ cell layers, disarray of germ cell layers, rupture of tubules, Leydig cell proliferation and reaction in the ruptured tubules, and oedema. RESULTS:There was a significant difference in the indicators of histological damage between groups 2 and 4 and groups 1 and 3, except for the Leydig cell reaction in the ruptured tubules and oedema. The damage was significantly less in group 3 than in groups 2 and 4. CONCLUSION:These results suggest that long-term histopathological changes in the contralateral testes are important after unilateral testicular torsion and that NO has a protective effect on the contralateral testis.