1. Two identical small plot experiments were done from 1961 to 1963, at the Hannah Dairy Research Institute, Ayr and at Boghall Farm, Midlothian to study the effects of applying nitrogen fertilizer at different times on total yield and its seasonal distribution. 2. Two perennial rye-grass plus white clover swards, one containing S23 and the other S24, were used at each site. On both swards a total of 10 cwt. of 舖Nitro-Chalk舗 (15ƃ5% N)/acre, applied at varying dates over the growing season, and a control treatment which received no nitrogen fertilizer were tested.