We have studied variation in mitochondrial protein and RNA synthesis during the development of a specialized rice (Oryza sativa L.) reproductive organ in a bacteria-free environment. Mitochondria were prepared from the maturing panicle during microsporogenesis when meiosis occurred and from etiolated seedlings at two growth stages. We found (1) that there was no discernible qualitative difference among the polypeptides synthesized by these three mitochondrial samples; (2) that the quantity of proteins synthesized by panicle mitochondria was approximately 3 times that of the seedling mitochondria, while the two seedling samples exhibited only a minor quantitative difference; (3) that panicle and seedling mitochondria samples synthesized qualitatively the same RNA but at distinctly different rates and that more RNA products were synthesized by panicle than by seedling mitochondria. These results, taken together, suggest that either the regulation of mitochondrial transcription and translation or the copy number of mitochondrial DNA per mitochondrion change discretely in the developing panicle and consequently that the level of mitochondrial gene expression increases considerably during the development of the reproductive structure in rice.