Mercury is ubiquitous in the environment; it is an occupational pollutant and a potential toxicant. We investigated the effects of exposure of rat testes to mercury vapor (Hg-0). Twelve male rats were divided into two groups of six: the rats of the Hg-0 group were exposed to mercury (1 mg/m(3)/day) in a chamber for six weeks; the control group rats were housed under the same conditions without exposure to Hg-0. After the experimental period, the testes were removed, sections of testis were evaluated histopathologically after hematoxylin and eosin staining, and stereologically using the Cavalieri principle and optical fractionator methods. We found significant decreases in the total volume of testis, diameters of seminiferous tubules and total volume of seminiferous tubules. Significant decreases were detected in the numbers of Sertoli cells, spermatogonia, spermatocytes and spermatids of the Hg-0 group compared to the control group. In the Hg-0 exposed group, spermatogenic cells were degenerated and seminiferous tubules were atrophied.