In a laboratory construction for heterotropic biological denitrification of drinking water treatment, the formation of nitrite, the potency of nitrosation and the genotoxic activity were tested. Parameter as nitrate concentration, the water flow rate in the system, nitrite and morpholine addition and the pH value were checked. For testing the potency of nitrosation ad formation of nitrite in the reactor we took Morpholin, a fast nitrosing amine. For the break down rate of nitrate, there was no influence of the initial nitrate concentration, the nitrite addition and the water flow rate in the system. pH values below 5.5 showed a little break down rate of nitrate. There was no correlation between the starting point of nitrate concentration and the formation of nitrite, although there was a positive correlation between the length of stay and the formation of nitrite. Nitrite and morpholine concentrations of 10 and 5 ppm showed no detectable formation of nitrosomorpholine.The analysis of different water tests in the construction did not show significant results for DNA damage by the sister-chromatid exchange (SCE).The results of the Salmonella microsome assay (tester strains TA98, TA100 with and without S9-mix) didn't show any mutagenic effects relating to the potency of nitrosation.According to our experiments the potency of generalising nitrosamides or nitrosamines by drinking water denitrification seems to be low. There is no final assessment of detriment to health by denitrifying drinking water.