The aim of the present study was the ecological monitoring of the natural waters in Stara Zagora Region, Bulgaria, establishing of a database and correlations regarding three basic parameters - nitrates, nitrites and ammonium, to provide a framework for sustainable management of the natural surface and groundwaters in the stated region. To accomplish this goal, the concentrations of nitrogen compounds in real surface and groundwater samples taken from 16 sampling points situated in four municipalities of Stara Zagora Region were spectrophotometrically determined and the statistical significance of the data was tested. The results obtained displayed that nitrite-nitrogen could be classified as a potential pollutant in the surface waters from Stara Zagora and Chirpan municipalities. NH4+-N concentrations were in the range of 0.118-0.462 mg L-1 in 6 of the samples, thus pertaining to II category surface waters. The comparative analyses proved that nitrate-nitrogen could not be classified as a contaminant. The derived exponential correlation between NO3--N and NO2--N concentrations in Tundzha River allows mathematical modelling of the results. The present study ascertained that the pollution thresholds for groundwaters, regarding nitrite and nitrates, were exceeded in three sampling points from Stara Zagora and Chirpan municipalities. (C) 2010 Elsevier B.V. All rights reserved.