A six-months continuous experiment was carried out on 48 viripotent white female rats, line "Vistar", divided into one control and three experimental groups of 12 rats each. The experimental groups received drinking water containing nitrates 50, 100 and 500 mg/dm3, respectively and the control group-7.0 +/- 0.03 mg/dm3. All animals were fed with a standard vivarium food containing 22.0 +/- 0.1 mg/kg nitrates. Upon expiry of the test period the animals were killed. Blood parameters and some internal organs were studied by macro- and microscopy. Microscopic changes in the thyroid gland, liver, kidneys, small and intestines, stomach of the test animals were established. The results of the blood analysis showed statistically significant deviations in haemoglobin values and the differential blood count as well as in some serum parameters.