This study investigated the oxidative stress induced after acute oral treatment with 500, 1000 and 2000 mg kg−1 doses of Al2O3-30 and −40 nm and bulk Al2O3 in Wistar rats. Both the nanomaterials induced significant oxidative stress in a dose-dependent manner in comparison to the bulk. There was no significant difference between the two nanomaterials. However, the effect decreased with increase with time after treatment. The histopathological examination showed lesions only in liver with Al2O3 nanomaterials at 2000 mg kg−1. Copyright © 2011 John Wiley & Sons, Ltd.