As a precursor of chlorophyll, 5-aminolevulinic acid (ALA) acts as a growth-promoting factor in plants, but little is known of molecular basis of plant growth action. The exogenous supply of an optimal concentration of 75 lM ALA without photooxidative risks, as indicated by a negligible change in conductivity, increased dry shoot biomass of rice up to 16 % after 10 days of ALA treatment, compared to the untreated control. Although rice plants treated with ALA gradually increased levels of ALA-synthesizing capacity during 30 h of the treatment, a noticeable increase in chlorophyll content was not observed. Levels of protoporphyrin IX and Mg-porphyrins were not significantly changed 6 h after ALA treatment and then decreased 30 h after the treatment, while transcript levels of most biosynthetic genes in tetrapyrrole pathway almost kept constant or slightly decreased. In microarray analysis of ALA-treated rice plants, categories of significantly upregulated transcripts highlight particular biological processes involved in cell cycle, transcription factors, posttranscriptional regulation, and metabolisms of macromolecules. Our results demonstrate that the regulation of tetrapyrrole biosynthesis did not contribute significantly to growth enhancement in ALA-treated rice. We propose that the elevated status of ALA in plant cells alters the transcriptional control of genes involved in physiological processes vital for plant growth, thereby contributing to enhancement of plant biomass.