INTRODUCTION: Nowadays, a lot of studies were carried out about side effects of nanoparticles especially nano-zinc oxide, but unfortunately, most of these studies have been conducted with high doses. MATERIAL AND METHODS: In the present study 36 male Wistar rats with approximate age of two months and weighing 300 ± 30 g were selected. After standard phases of preparation, injections were done intraperitoneally in chronic groups with zinc oxide nanoparticles (20 and 40 mg/kg) every other day and for acute groups with 200 and 400 mg/kg in a single dose. After the determined time for every group (21 days for chronic groups and 7 days for acute groups), blood samples were provided from rat hearts and also liver, kidney and testicular tissue were removed for histological studies. RESULTS: There were signifi cant decreases in lymphocytes and platelet counts and signifi cant increases in white blood cells count, hemoglobin and hematocrit in acute dose compared to control group and no difference in red blood cells count in all groups. A severe to slight damage was observed in liver tissue in all groups and tissue destruction was seen in kidney only in acute dose treated groups but no specifi c pathological effect in testis was observed (Fig. 8, Ref. 43). Text in PDF www.elis.sk.