Aims: the present study was performed to investigate the hematological and immunohistopathological changes in the testes of experimental rats after exposure to formaldehyde. Method: Twenty-four Albano Waster male rats weighing (150-170g) were divided into four groups (6 rats for each group). The control group was not exposed to the formaldehyde vapor. In group two the rats were exposed directly to formaldehyde vapor with a mean concentration of 1.5 ppm (1 hour daily) for 20 days in a special chamber. In group three the rats were exposed directly to formaldehyde vapor with a mean concentration of 1.5 ppm (2 hours daily) for 20 days in a special chamber. In group four the rats were exposed directly to formaldehyde vapor with a mean concentration of 1.5 ppm (3 hours daily) for 20 days in a special chamber. After the experiment ended, blood samples were collected from the rats for hematological parameters. Testes organs were immediately removed from experimental rats for immunohistochemical examination and KI67 immunoreactivity was performed using an Avidin-Biotin-Peroxidase (ABP) immunohistochemical method with KI67 monoclonal antibody (dilution 1:100; DAKO Japan Co, Tokyo, Japan). Results: Exposed groups showed increased testes weight, decreased red blood cell parameters and platelets, and increased white blood cells. Ki-67 immunoreactivity decreased with exposure duration, along with structural damage to seminiferous tubules and inflammatory cell accumulation in longer exposures. Conclusion: Formaldehyde vapor exposure, even at low doses, can cause hematotoxicity and detrimental effects on testicular tissue structure and cell proliferation.