The origin of NO yielded in drainage from agricultural grasslands is of environmental significance and has three potential sources; (i) soil organic mater (SOM), (ii) recent agricultural amendments, and (iii) atmospheric inputs. The variation in 15N-NO and 18O-NO was measured from the inter-flow and drain-flow of two 1 ha drained lysimeter plots, one of which had received an application of 21 m3 of NH-N-rich agricultural slurry, during two rainfall events. Drainage started to occur 1 month after the application of slurry. The concentrations of NO-N from the two lysimeters were comparable; an initial flush of NO-N occurred at the onset of drainage from both lysimeters before levels quickly dropped to