The aim of this study was to determine the environmental concentration of atrazine (ATZ) in five streams located in the north of São Paulo state (Brazil) and evaluate its toxicological impact on young specimens of the pacu fish Piaractus mesopotamicus. Samples of water were collected on three occasions between 2010 and 2011, corresponding to periods signifying the beginning, middle, and end of rain season. ATZ levels were estimated by a high-performance liquid chromatography coupled with a mass spectrometry (HPLC-MS/MS) triple quadrupole. Later, the quotient of environmental risk (QR) was determined based on the medium lethal concentration (LC 48 h), non-observable effect concentration (NOEC), and the estimated environmental concentration (EEC) of ATZ detected in the environment. Histological changes in gills and liver were also studied, along with the brain activity of the enzyme acetylcholinesterase (AChE). The highest concentration of ATZ measured was 10.4 μg L . The ATZ LC50 (48 h) for young P. mesopotamicus was 24.46 mg L and the QR was classified as “safe”. Although the QR indicated that ATZ could be safe for the tested species, it caused many histological alterations in the liver and gills of the exposed specimens, and an increase in the AChE levels. Predmet ovoga istraživanja bio je odrediti okolišne koncentracije atrazina u pet vodotoka na sjeveru brazilske savezne države São Paulo te ocijeniti njihove toksikološke učinke na ribu Piaractus mesopotamicus. Uzorci vode prikupljeni su u tri navrata između 2010. i 2011. godine, i to na početku, usred i pri kraju kišne sezone. Razine atrazina mjerene su trostrukim kvadrupolom s vezanim sustavom tekućinska kromatografija visoke djelotvornosti-spektrometrija mase (HPLCMS/ MS). Potom je izračunat kvocijent okolišnog rizika (QR) na temelju srednje smrtonosne koncentracije (LC 48 h), maksimalne koncentracije bez učinka (NOEC) te procijenjene okolišne koncentracije (EEC) atrazina pronađenog u okolišu. Nadalje, istražene su histološke promjene u škrgama i jetri te izmjerena aktivnost enzima acetilkolinesteraze (AChE) u mozgu. Najviša izmjerena razina atrazina bila je 10.4 μg L . LC50 (48 h) atrazina za mlade jedinke P. mesopotamicus bio je 24.46 mg L , a QR je klasificiran kao „siguran“. Premda QR upućuje na to da bi se atrazin mogao okarakterizirati sigurnim za ispitanu vrstu, uzrokovao je ne samo brojne histološke promjene u jetri i škrgama izloženih jedinki, nego i porast razina AChE u mozgu.