In field trials in 1971–73, perennial ryegrass cv. S23 and S24 were given up to 200 kg ha−1 nitrogen (N) and dry weight, seed yields and seed yield components measured. Optimum levels of applied N ranged from 80 kg ha−1, where livestock had grazed the crop in the establishment year or where residual N levels were about 70 kg ha−1, to 120 kg ha−1 where residual N was low. Application of more than 120 kg ha−1 did not increase yields further because of increased lodging and increased production of vegetative tillers. Seed set was 37–55% in S24 and 25–29% in S23 and was decreased by lodging.