Abstract: A pot culture experiment was conducted at the Tamil Nadu Agricultural University, Coimbatore, to study the Nitrogen Use Efficiency (NUE) of rice under different eco-systems. The different eco-systems of rice cultivation evaluated in the study were i) direct seeding of sprouted seeds under puddled condition, direct seeding of ii) sprouted and iii) dry seeds under unpuddled condition and iv) transplanting of rice seedlings under unpuddled condition. In the puddled soil condition pots, 5 cm depth of water was maintained throughout the crop growing period. In respect of unpuddled condition pots, 5 cm depth of irrigation was given at 80% of the available soil moisture condition. The nitrogen levels were 50, 75, 100, 125 and 150% of recommended dose of nitrogen. Nitrogen tracer (15N) technique is an accurate tool to assess the absorption and distribution of added N in the soil crop environment, as influenced by the nitrogen levels. Assaying on 15N tracer added as labelled urea in rice brought out valid information in the present study on quantity of N derived as crop recovery (N use efficiency). The N use efficiency under 15N study was higher in direct seeding of sprouted seeds under puddled condition. Among the different N levels, increasing the N level up to the recommended level (100%), the N use efficiency was increased. Further, increasing the N levels to 125 and 150%, the N use efficiency was decreased under all the systems of cultivation. It may be due to the application of excess nitrogen, which was not effectively utilized by the crop and the rate of production was lesser per unit of N application.