Previous studies have shown that toxins produced by toxic cyanobacterial blooms are hazardous materials. In the present study, 1 mu g/L microcystin-LR (MC-LR) was observed to induce apoptosis in the testes of male Rana nigromaculata via the mitochondrial and endoplasmic reticulum (ER) pathways at exposure times ranging from 7 d to 14 d. The results showed that reactive oxygen species production and malondialdehyde content were positively correlated with exposure time. Antioxidant enzyme contents, such as reduced glutathione and glutathione peroxidase rapidly decreased, implying that the defense system of the testes induces oxidative damage. MC-LR significantly stimulated the release of cytochrome c in the testes, thereby improving the protein expressions of Bax and caspases-3, 8, and 9 (p < 0.01) and inhibiting the protein expression of Bcl-2 with prolonged exposure (p < 0.01). Ultrastructural observations showed distention of the mitochondria and endoplasmic reticulum and deformation of the nucleolus. Moreover, prolonged exposure times strengthened and weakened the relative expression levels of C/EBP homologous protein and GRP78, respectively. These results indicate that MC-LR-induced apoptosis of the testes in male frogs in vivo may occur through the mitochondrial and ER pathways. It also further proves our previous findings that MC-LR can induce toxicity in the male reproductive system of R. nigromaculata in vitro. The findings show that MC-LR is highly hazardous to frogs and that the accepted drinking water limit of 1 mu g/L MC-LR exerts significant toxicity to amphibians. (c) 2013 Elsevier B.V. All rights reserved.