The conversion of the original Atlantic Forest into areas for agriculture, planted forests, cattle and other activities have caused environmental impacts whose consequences have not yet been completely evaluated scientifically, including hydrological studies in places where eucalyptus forests had been planted. . In this context, the main objective of the present paper was to carry out an hydrological study in a micro-basin covered with eucalyptus forest, near Parque Estadual da Serra do Mar(State Park), Santa Virginia nucleus. The study was performed from January to December of 2008 and the results showed that the precipitation was 1290 mm, where 14% was intercepted by the vegetation, 38% were converted in streamflow and the evapotranspiration was estimated as about 794 mm. The surface runoff was of little significance, due to a low intensity of the precipitation