Two levels of nitrogen application (70 and 180 lb per acre per annum) were compared on 6 long‐duration leys. The treatments were repeated for periods of up to 4 years on tiie same swards. The higher N dressing produced 2230 lb more dry matter per acre than the low rate, an increase of 28%, and the yield was also more evenly spread throughout the season. There was virtually no clover on the high‐N swards. The proportion of clover varied under low N but tended to increase as the season progressed. The sown grasses remained the dominant sward constituents throughout the experiment, and there was no sign of loss of vigour on the high‐N swards. Utilized‐starch‐equivalent outputs were calculated from 3 different swards for 1956–8 and there was a significant positive correlation between the dry‐matter yields and utilized‐starch‐equivalent