To determine the source of dissolved inorganic nitrogen (N) in runoff, approx. 35kg N enriched with the stable isotope 15 N (2110‰ δ 15 N) was added to a mature coniferous forested catchment for one whole year. The total N input was approx. 50kg ha −1 year −1 . The enrichment study was part of a long-term whole-catchment ammonium nitrate addition experiment at Gårdsjön, Sweden. The 15 N concentrations in precipitation, throughfall, runoff and upper forest floor were measured prior to, during, and 3–9years following the 15 N addition. During the year of the 15 N addition the δ 15 N level in runoff largely reflected the level in incoming N, indicating that the leached NO 3 − came predominantly from precipitation. Only 1.1% of the incoming N was lost during the year of the tracer addition. The cumulative loss of tracer N over a 10-year period was only 3.9% as DIN and 1.1% as DON.