All forms of mercury are considered poisonous. Methylmercury, one organic form, is highly toxic to many organs. The aim of the present study was to assess the effects of this form on the reproductive system in the rat. For this, 20 male rats were divided into two groups. One, which is considered as reference, received tap water. The second group received tap water containing methylmercury at the rate of 20 mg l⁻¹ for 8 weeks. At the end of the experiment, blood samples were collected for the determination of total mercury and plasma testosterone. The left testes were used for the determination of total mercury and histological examination. Appropriate centrifugation was applied on right testes to extract interstitial and seminiferous tubular fluids. The epididymides were homogenised for the sperm count. Our results showed a dramatic fall in the plasma testosterone in the contaminated animals. The fall in plasmatic testosterone seems to be in relation with the decrease in the secretion of testosterone. In association with this, the concentration of testosterone in seminiferous tubules fluid dropped about 55% in the poisoned animals in comparison with the controls. Despite this, no decrease in the epididymal sperm count in contaminated rats was observed.