The establishment of a water balance for the Amazon Basin constitutes a problem of difficult solution, not only on the account of its extension and characteristics, but also for lack of sufficient meteorological and hydrological data. In an attempt to estimate the magnitude of the main components of the water balance, a study was made with data from the Brazilian Amazon Region and from some observation stations in other countries. An energy balance was made and based on this balance the water balance of the region was established, having the Penman method been adapted to forest conditions. The data obtained indicate that 90% of the evapotranspiration is due to the energy balance. The evapotranspiration in this area should be very close to the potential evapotranspiration, and the average found was of the order of 4mm/day, i.e., 1460mm/year. As a first approximation it was found that the Amazon Basin system receives 12 x 1012m3 water/year through precipitation, this total being balanced by a surface discharge of 5,5 x 1012m3/year and an evapotranspiration of 6,48 x 1012m3/year. Since transpiration by plants represents 54% of the precipitacao, all seems to indicate that intensive deforestation shall bring about alterations of the hydrologic cycle.