SummaryPerennial ryegrass growing in monolith lysimeters and treated with 400 kg N ha‐1as calcium nitrate labelled with nitrogen‐15 (10.5 atoms per cent), during one growing season recovered between 43 and 54 per cent of the fertilizer nitrogen. In the following year without further nitrogen additions 4.6–9.5 per cent was taken up, whilst in the fifth year the recovery was less than 1 per cent. The contribution of non‐fertilizer sources of nitrogen to the total nitrogen taken up by the plants during the season that nitrogen was applied was estimated using tracer methods to be about 13–14 g N m‐2year‐1. The estimate from measuring the nitrogen content of an unfertilized sward was 7 g N m‐2year‐1. The residual effects of a fertilizer application are likely to be detectable for a period of between 6 and 9 years.Losses of nitrogen to drainage in the winter after application represented 2–5 per cent of the fertilizer applied, whilst in subsequent years the amounts did not exceed 0.1 per cent. Mean concentrations of nitrate ranged between 4 and 16 mg N I‐1. Fertilizer contributed about 60–70 per cent of the total nitrogen lost in the first winter after nitrogen application and 45–60 per cent av