mRNA expression patterns of genes for metabolic key enzymes sucrose phosphate synthase (SPS), phosphoenolpyruvate carboxylase (PEPC), pyruvate kinase, ribulose 1,5-bisphosphate carboxylase/oxygenase, glutamine synthetase 1, and glutatmine synthetase 2 were investigated in leaves of rice plants grown at two nitrogen (N) supplies (N(0.5), N(3.0)). The relative gene expression patterns were similar in all leaves except for 9(th) leaf, in which mRNA levels were generally depressed. Though increased N supply prolonged the expression period of each mRNA, it did not affect the relative expression intensity of any mRNA in a given leaf. SPS V(max) SPS limiting and PEPC activities, and carbon flow were exartimed. The ratio between PEPC activity and SPS V(max) was higher in leaves developed at the vegetative growth stage (vegetative leaves: 5(th) and 7(th) leaves) than in leaves developed after the ear primordia formation stage (reproductive leaves: 9(th) and flag leaves). PEPC activity and SPS V(max) decreased with declining leaf N content. After using (14)CO(2) the (14)C photosynthate distribution in the amino acid fraction was higher in vegetative than in reproductive leaves when compared for the same leaf N status. Thus, at high PEPC/SPS activities ratio, more (14)C photosynthate was distributed to the amino acid pool, whereas at higher SPS activity more (14)C was channelled into the saccharide fraction. Thus, leaf ontooeny was an important factor controlling photosyntliate distribution to the N- or C-pool, respectively, regardless of the leaf N status.