The aim of this study was to investigate whether microcystin-LR, one of the most common cyanobacterial toxins has toxic effects on reproductive system in vivo or Leydig cells in vitro. Male rats were treated with MC-LR (i.p.) at a dose of 0, 5, 10 or15 microg/(kgday) for 28 days. Leydig cells were cultured with a culture medium including 0, 0.5, 5, 50 or 500 nM MC-LR. In vivo study, we observed exposure to 5 microg/(kgday) of MC-LR decreased the sperm motility, increasing the sperm abnormality rate, 15 microg/(kgday) of MC-LR led to the decrease of testis weight and sperm concentration, decreased the levels of serum testosterone, FSH and LH. The histological findings showed that the seminiferous tubules atrophied and obstructed. In vitro study evaluated MC-LR-induced toxicity and oxidative stress in Leydig cells. It was observed 50 and 500 nM MC-LR significantly decreased the cell viability, increasing the apoptotic DNA fragmentation, and increasing the ratio of necrotic cells. The Leydig cells exposed to MC-LR decreased testosterone production. 500 nM MC-LR increased ROS production, 50 or 500 nM MC-LR enhanced the lipid peroxidation. All Leydig cells exposed to MC-LR showed decreased SOD activity. The results of this study showed that the oxidative stress of MC-LR might lead to cytotoxicity, which may play an important role in cell apoptosis. Then could reduce the production of testosterone in Leydig cells and result in reproductive toxicity.