The gills of fish are biomarkers of environmental toxicity which are often used to assess the impacts of xenobiotics. This study aims to evaluate the contamination of the reservoir Marechal Mascarenhas de Moraes, through the estimation ofthe rate of morphological changes in the gills of Pimelodus maculatus caused by toxic agents. The specimens of P. maculatus were collected from three points of the reservoir of UHE Marechal Mascarenhas de Moraes. In the local of collection, immediately, the gills of the collected individuals were removed and fixed in Bouin. They were stained with Hematoxylin-Eosin for light microscopic analysis. The quantification of changes was carried out through stereology and the indices of change were estimated, classifying the lesions from moderate to severe in the three points under study.