The yield results are reported for the final 3 years of a 6-year experiment in which twenty-one rates of nitrogen fertilizer ranging from 0 to 800 lb/acre (897 kg/ha) were applied annually on S. 23 perennial ryegrass swards with and without S. 100 white clover. The long-term effects of the treatments on herbage yields are discussed using the results from all 6 years. Four-parameter exponential curves fitted to the herbage yield data from each year show that the response pattern for the pure-grass sward remained relatively constant over the years although the mean annual yields varied slightly. The response of dry-metter yield to nitrogen rate on this sward was almost linear between the 0 and 300 lb nitrogen/acre (336 kg/ha) rates, then it decreased progressively, becoming non-significant about the 500 lb/a ere (560 kg/ha) ra te. Crude-protein yield responded almost linearly up to the 600 lb/acre (673 kg/ha) rate. The average yield responses over the 6 years for the pure-grass sward with nitrogen rates between 0 and 300 lb/acre was 27–8 lb dry matter (range 25·8–29·8 lb) and 4·2 lb crude protein (range 3·8–4·8 lb) per lb of nitrogen applied. At high nitrogen rates the yield curves for the grass and clover sward were similar to those for the pure-grass sward but at low rates the grass and clover sward gave a lower response to nitrogen and a greater yield at each nitrogen rate. The yield response to nitrogen rates between 0 and 300 lb/acre applied on this sward increased from 13·1 lb dry matter/lb of nitrogen in the first year to 20·2 lb in the sixth year. The difference in yield between the grass and clover and the pure-grass swards given no nitrogen fertilizer decreased from 4850 lb dry matter/acre (5440 kg/ha) to 2370 lb/acre (2660 kg/ha) over the same period. Within each year the yield difference between the two swards decreased with increasing nitrogen rate reaching an insignificant value at a rate of about 350 lb/acre (392 kg/ha) in the first year declining to 150 lb/acre (168 kg/ha) in the sixth year. On average the weight of nitrogen required annually on the pure-grass sward to give the same herbage yield as the grass and clover sward receiving no nitrogen was 125 lb/acre (140 kg/ha) on a drymatter basis and 195 lb/acre (219 kg/ha) on a crude protein basis.