In addition to their production function, grasslands act on the enviroment in ways such as the protection of the water quality and of the soil quality. How far and under what conditions can these non-productive functions be carried out by perennial grass covers ? The main processes are described that are implied in the nitrogen and the phosphorus cycles, followed by a reminder of the means of limiting the N and the P losses, at the level of the field, of the farm, and of the catchment basin. The land-use pattern (with the proportion of perennial forage covers) is of great importance. The environmental performances of the grasslands are reported for various types of forage systems. It is not easy to weight the different aspects of the water quality that would favour most adequately the effects of the grasslands (areas and management) where the environmental risks are large. It is absolutely necessary to take into account several dimensions (territorial, temporal, congitive) and to allow for the multiplicity of the problems involved.