Technical Abstract: The amount of nitrogen (N) accumulating in rice grains will depend on the ability of plants to utilize N derived from two sources: N reserves already present within vegetative tissues (endogenous N) and N uptake and assimilation occurring during grain development (exogenous N). However, it is still unclear to what extent various N sources (both temporal and spatial) provide N for developing gains, or how this partitioning might regulate final seed protein content. In the present study, 'Lemont' rice was grown in a recirculating hydroponic system with NO3- as the N source. To monitor N uptake and mobilization, plants were supplemented with 7% 15N- enriched NO3- from anthesis to grain maturity. Plants were harvested at 0, 7, 14, 21, 28 and 35 days after anthesis and immediately freeze dried. Total N and atom% 15N were determined by a Continuous-Flow Isotope-Ratio MS. Nitrate uptake occurred throughout grain development, and both vegetative and reproductive organs accumulated exogenous N. Approximately 70% and 30% of grain N was derived from endogenous and exogenous pools respectively. Secondary tillers, sheaths and leaf tissue were the primary sources of endogenous N for the grains. However, total N cycling within these tissues fluctuated throughout grain development, with organs initially acting as N sources during the first half of grain development, and then becoming N sinks near grain maturity. Additional studies also are being done to determine whether the exogenous N present in tissues during grain development was reduced and utilized for plant growth and development or whether it was stored in tissues as NO3-. This research was funded by the USDA-ARS under Coop. Agr. No. 58-6250-1-003.