The present investigation aimed to elucidate possible mechanisms of aluminum (Al)-induced reproductive toxicity in male mice. Daily intraperitoneal administrations of Al chloride (0, 1/8 and 1/3 LD(50)) were conducted for 12 or 16 days, followed by a 2-week withdrawal period. Serum Al levels significantly increased in Al-treated animals, compared to controls, whereas serum testosterone concentrations were markedly decreased. In addition, significant increases in nitric oxide products (NO(x)) were also observed during Al injection. Similar trends were found for testicular Al, NO(x) and testosterone levels. However, during the withdrawal period, when diminished testicular Al levels were observed, the changes in NO(x) and testosterone levels were indistinguishable from control values. It was concluded that: (i) Al exerted a significant adverse effect on the steroidogenesis; (ii) the process was reversible; and (iii) increased production of NO(x), induced by excessive Al, might inhibit testosterone levels.