The analysis of river flow trends can help in water planning and in understanding the hydrological behavior. This study aims to analyse flow trends in Brazilian river basins, using the Mann-Kendall method. 16 fluviometric stations, spread over the main Brazilian basins, were analysed. The results show that positive annual trends were observed in the flow of 75% of the fluviometric stations in the South of Brazil and in the South portion of the Southeast; and negative annual trends in 50% of the stations in the North portion of the Southeast, Northeast and North of the country. Similar results were detected at the quarterly flow rates. It was also observed a tendency to intensify the extremes, mainly in the South and Northeast of Brazil, which can generate or worsen conflicts in the use of water or increase the occurrence of floods or droughts. Climate variability and human actions are factors that may have contributed to the changes observed in the river flows.