The aim of this studywas to investigate whether microcystin-LR, one of the most common cyanobacterialtoxins has toxic effects on reproductive system in vivo or Leydig cells in vitro. Male rats were treated withMC-LR (i.p.) at a dose of 0, 5, 10 or15_g/(kg day) for 28 days. Leydig cells were cultured with a culturemedium including 0, 0.5, 5, 50 or 500nM MC-LR. In vivo study, we observed exposure to 5_g/(kg day)of MC-LR decreased the sperm motility, increasing the sperm abnormality rate, 15_g/(kg day) of MC-LRled 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. Invitro study evaluated MC-LR-induced toxicity and oxidative stress in Leydig cells. It was observed 50 and500nM MC-LR significantly decreased the cell viability, increasing the apoptotic DNA fragmentation, andincreasing the ratio of necrotic cells. The Leydig cells exposed to MC-LR decreased testosterone production.500nMMC-LR increased ROS production, 50 or 500nMMC-LR enhanced the lipid peroxidation. All Leydigcells exposed to MC-LR showed decreased SOD activity. The results of this study showed that the oxidativestress of MC-LR might lead to cytotoxicity, which may play an important role in cell apoptosis. Then couldreduce the production of testosterone in Leydig cells and result in reproductive toxicity.