The absorption rate of nitrogen (N) fertilizer labelled with ('15)N was studied during the flowering and fruit set periods in Citrus mitis Bl., trees grown in sand culture. The absorbed N was concentrated preferentially in ovaries, young fruits, new flush leaves and new flush twigs, but N supplied to these organs comes mostly from the N reserve in old leaves, old twigs and stem, and roots, and less than 3% of the absorbed N reached ovaries and young fruits. Nitrogen translocation to young fruits was higher during the fruit set period. Spring leaves were an essential organ since they received a large portion of the absorbed N and were quickly converted into a supplying organ. Old leaves, old twigs and stem, and roots supplied a great deal of N to developing organs; during fruit set, their N supply was replenished with N coming from fertilizer. During flowering, trees absorbed 30 mg per 1000 g of whole tree (dry weight) and per day. During fruit set, this amount increased considerably.