Seasonal and interannual variations in nitrous oxide (N2O) losses from agricultural soils hamper the accurate quantification of the N2O source strength of these soils. This study focuses on a quantification of seasonal and interannual variations in N2O losses from managed grasslands. Special attention was paid to N2O losses during the growing season and off-season as affected by grassland management. Fluxes of N2O from grasslands with three different types of management and on four different soil types in the Netherlands were measured weekly during two consecutive years, using flux chambers. There were distinct seasonal patterns in N2O losses, with large losses during spring, summer, and autumn but relatively small losses during the winter. These seasonal variations were related to fertilizer N application, grazing and weather conditions. Measurements of N2O concentrations in soil profiles showed that a rise in groundwater level was accompanied by increased N2O concentrations in the soil. Disregarding off- season losses would underestimate total annual losses by up to 20°being largest for unfertilized grassland and smallest for N-fertilized grazed grassland. Total annual N2O losses ranged from 0.5 to 12.9 kg N ha-1 yr-1 for unfertilized grasslands to 7.3 to 42.0 kg N ha-1 yr-1 for N- fertilized grazed grasslands. Despite the considerable interannual variations in N2O losses, this study indicates that the results of measurements carried out in one year have predictive power for estimating N2O losses in other years.