The effects of nitrogen supply and growth period on yield and quality of herbage were studied. It was shown that internal nitrate content was a good measure of the nitrogen status of herbage. Maximum output from grassland could not be obtained if internal nitrate concentration in the herbage remained below about 100 mmol per kg dry matter. This figure held for dressing with ammonium nitrate, limestone and calcium nitrate. It did not apply to application of ammonium sulphate except where conditions favoured nitrification. Ageing was accompanied by a decrease in total N and an increase in crude fibre. In young herbage, crude fibre decreased with N dressing (if yield had not yet increased considerably by fertilizer), but increased in old herbage when the yield had been increased considerably by fertilizer. At one yield level, crude fibre always decreased with increasing rates of nitrogen. Both growth period and nitrogen fertilizer affected the botanical composition of herbage and the morphological composition of the herbage components. Grass leaf had more crude fibre than dandelion leaf and leaves were lower in crude fibre than stems.