Influences of flowering intensity and spring weeds on ¹⁵N uptake by citrus trees were examined with Miyauchi-iyo (Citrus iyo hort, Tanaka) and Satsuma mandarin trees. In highly flowering Miyauchi-iyo trees, total nitrogen contained in fallen flowers accounted for more than 10% of the nitrogen in the fruits harvested in autumn. The fruit setting under leafy inflorescences was improved by nitrogen supply prior to anthesis. The ovary growth rate during the period of 4 to 5 days before the full-blooming stage was greater in leafy inflorescences than in leafless ones. The ¹⁵N applied in soils in early September was absorbed and translocated into shoots, leaves and fibrous roots, and the contents remained almost constant during the winter season. However, in the period of March to May, the ¹⁵N content in the old plant organs decreased concurrently with an increase in its content in new leaves and flowers. The ¹⁵N applied in soils prior to anthesis (mid April) was absorbed and uniformly distributed to new shoots and flowers at anthesis. Abscised flowers contained a considerable amount of ¹⁵N. This indicates that a great deal of nitrogen was lost through flower falls. On the other hand, the nitrogen applied in soils at the beginning of flowering (early May) had little effect on fruit setting, but it enhanced the development of new shoots after anthesis. A larger amount of ¹³C-photosynthates were translocated into leafy inflorescences than into leafless ones. The ¹³C contents in abscised flowers were rather low. Thus, it is presumed that nitrogen supply promotes translocation of photosynthates to flowers, resulting in an increase of fruit setting. Spring weeds weighed 32.3 t/ha. Total amount of nitrogen in the weeds was 105 kg/ha, which was equivalent to 38.6% of the ¹⁵N applied in early spring. When the cut weeds were mulched, nearly 75% of the ¹⁵N disappeared by September. This nitrogen was partly transferred to soils and partly reabsorbed by the citrus trees. As a consequence, in the weed plot, the ¹⁵N contents in leaves and fruits of the trees increased in September and beyond, while in the open ground plot, the ¹⁵N contents in flowers and leaves were higher in early summer.