This study was designed to investigate the effect of ATP-MgCl2 administered before and after detorsion on the prevention of reperfusion injury after unilateral testicular torsion. The rats were divided into six groups, each containing six rats. Torsion was created by rotating the left testes 720 degrees in a clockwise direction. Group 1 functioned as a control group. Torsion only was carried out in Group 2. Detorsion was carried out in Group 3. ATP-MgCl2 (100 mu mol/kg) was injected intravenously immediately before detorsion in Group 4. ATP-MgCl2 (100 mu mol/kg) was injected intravenously immediately after detorsion in Group 5. Saline was injected intravenously immediately after detorsion in Group 6. The effect of ATP-MgCl2 on reperfusion injury was investigated by determining the levels of thiobarbituric acid-reactive substances (TBAR) and resulting lipid peroxidation in the bilateral testicular tissue. Testicular torsion and detorsion caused a significant increase in the TBAR levels in the bilateral testicular tissue. TBAR levels decreased to approximately normal levels in Groups 4 and 5. It is concluded that if reperfusion injury has occurred in both testes after unilateral testicular torsion, ATP-MgCl2 administered before or after detorsion may prevent reperfusion injury in testicular torsion.