ABSTRACT: Recently (mainly in the past 30 years) the concentrations of nitrogen compounds, especially in the form of nitrates have been increasing dangerously in drinking water, but elsewhere as well. This has been caused by sewage and some other industrial waste waters, as well as mineral nitrogenous fertilizers that are being fIushed away from agricultural land, further by agricultural wastes (urea, ensilage wastes, etc.), precipitation and nitrogen being liberated from the subsoil. However, the main sources of nitrates are first of all agricultural production and waste water. The harmful effect of nitrates is indirect. It can be observed only in the human body, where nitrates are reduced to toxic nitrites NO2- in the alimentary canal (the maximum admissible value of NO2- in drinking water amounts 0.1 mg/l). The Slovak drinking water standard determines the maximum value of NO3- being 50 mg/l. However, the recommended value is much lower, 15 mg/l NO3-. The maximum value is based on the assumption that an adult should not take in more than 350 mg of NO3- daily in the form of liquids and food, whereby an average adult should not take in more than 250 mg of NO3-. in the form of food and provided that the average daily water demand amounts to approximately 2 l, either in the form of drinking water and beverages or contained in food, the other 100 mg of NO3-.should be taken in the form of liquids. If the maximum admissible daily limit of 200 mg NO3- is being exceeded over a longer period of time, various diseases may occur (especially carcinogenic diseases). In order to prevent carcinogenic diseases of the alimentary canal it is necessary to reduce the amount of nitrates taken in by consuming unfit agricultural products as well as to ensure that clean drinking water is being distributed to the consumer.