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In rice (Oryza sativa L.) plants, the leaf sheaths of the upper leaves accumulate a large amount of starch before heading, and the accumulated starch is converted to sucrose and translocated to the panicles after heading. To analyze the regulation of sink-source transition, we carried out large-scale monitoring of gene expression by microarray analysis using 8987 rice expressed sequence tags (ESTs), and identified 102 developmentally regulated genes in the leaf sheaths of the first leaves below the flag leaves during the heading period. Identified genes included multiple genes related to starch biosynthesis, cell division and expansion, and photosynthesis. All of them showed early stage-preferential expression, probably reflecting the decrease of starch biosynthesis, end of elongation and decrease of photosynthesis in the leaf sheaths during the heading period, respectively. The expression patterns of the genes for starch biosynthesis enzymes and α-tubulin suggest that the leaf sheaths displayed both accumulating- and consuming-sink functions at the very early stage of the heading period. Northern blot analysis of the genes for starch degradation and sucrose biosynthesis enzymes (α-amylase3D and sucrose phosphate synthase) revealed that the induction of the genes did not occur when the starch amount began to decrease, suggesting that the mechanisms of starch degradation and sucrose re-synthesis in the leaf sheaths during the heading period were different from those in germinating seeds. Furthermore, we identified 18 developmentally regulated genes whose functions in the leaf sheaths are unknown. They included seven genes those were preferentially expressed in the bottom part of the leaf sheaths, implying that they were involved in functions related to the starch metabolism. The results obtained showed the effectiveness of the microarray technique to analyze complex and uncharacterized phenomena.