〔Abstract〕 Objective To study the pathological mechanism of ornidazole inducing asthenospermia and its influences on spermatic mitochondrial function in rats. Methods Forty male SD rats were randomly divided into control group, low-dose, medium-dose and high-dose ornidazole group, 10 cases in each group. All the ornidazole groups were given 200, 400 and 800 mg/(kg·d) ornidazole for gavage, while control group was given the same amount of sodium carboxymethyl cellulose solution for 4 weeks. The general conditions in gavage process in each group were observed. At 30 minutes after the last administration, decapitation was conducted to collect unilateral testis and epididymis so as to calculate visceral indexes. The concentration and vitality of sperm were detected. And some epididymis and testis were preserved for light microscopy and electron microscopy. The other unilateral testes were applied to make tissue homogenate for detecting the activities and levels of acid phosphatase(ACP), alkaline phosphatase(ALP), lactate dehydrogenase(LDH), succinate dehydrogenase(SDH), superoxide dismutase(SOD) and malondialdehyde(MDA). The membrane potential changes of sperm mitochondria were detected by JC-1 staining. Results Compared with control group, body mass gain, testis, visceral indexes, sperm concentration, survival rate and activity of epididymis decreased after ornidazole treatment, showing concentration-dependence(P<0.05). Compared with control group, tissue morphology of testis and epididymis was damaged after ornidazole treatment. The higher the ornidazole dose was, the more severe the damage was. Compared with control group, activities of ACP, ALP, LDH, SDH and SOD decreased after ornidazole treatment, while MDA level increased, showing concentration-dependence(P<0.05). Compared with control group, ultrastructure of sperm mitochondria was damaged to different degrees under electron microscope after ornidazole treatment, and normal ratio of membrane potential also decreased. The changes in mitochondrial damage and normal ratio of membrane potential were the most significant after high-dose ornidazole treatment(P<0.05). Conclusion Ornidazole may destroy mitochondria structure by regulating energy metabolism and oxidative stress, thus leading to reproductive toxicity.