The water quality of forest brooks was monitored after glyphosate application to two Norwegian forest areas. The application was done according to common forestry practice. The major effect on water quality of glyphosate is to increase the concentration of NO3−. The maximum concentration found was 80–90 μM NO3−the first and second winters after application. Without glyphosate application, the level was estimated to be 20–40 μM lower. Generally the increase in NO3−was smaller or undetectable. Only at the higher levels of NO3−was an increase in water acidity observed. This occurred the first summer after application and was in the range 1–3 μm H+. No other effects on the water quality resulting from glyphosate application were detected. Glyphosate application as part of common forestry management in Norway gives only small changes in the water quality of forest brooks. As long as small areas of the forest are treated the consequences of the changes in water quality appear nonsignificant.