[Show abstract][Hide abstract] ABSTRACT: A field experiment with perennial ryegrass (Lolium perenne L.) grown for seed was conducted in Mochełek, at the Research Station of Agriculture Faculty of University of Technology and Life Sciences in Bydgoszcz. The aim of the study was to analyse the effect of nitrogen fertilization doses and the length of use of perennial ryegrass lawn cultivar grown for seed on biological characters, their variation and interrelationship. Forming numerous and relatively long tillers and ears, as well as well-formed flag leaves, resulted in increasing seed mass per ear and seed yield. Every additional 30 kg N•ha-1 within the range from 0 to 90 kg•ha-1 stimulated production of seed mass per ear. Increasing the dose from 30 to 60 kg N•ha-1 affected the growth of tiller number and ear length, and the next 30 kg N•ha-1 increased the length and width of flag leaf blade. Plant ageing was accompanied by decreasing tiller length, seed mass per ear, the length of tillers and ears. Of biological characters tested, the number of tillers showed the highest variation, and the length of use had three times more influence on the variety of this character than nitrogen fertilization level.