The activity of superoxide dismutase (SOD) and glutathione peroxidase was marked in homogenates of rat's (type Wistar) liver after 1, 2 and 3 months of watering them with 0.02% and 0.04% solution of sodium nitrate in the situation when the rats' diet was supplemented with vitamin E. In comparison with the control group it was stated that the activity of SOD increases and the activity of GPx decreases in liver homogenates sampled from rats watered with nitrate solutions. In liver homogenates sampled from rats which apart from being watered with nitrates were also administered vitamin E, a relative normalisation of the activity of both enzymes was stated. The obtained results indicate direct or indirect free- radical character of the nitrates effect.