EM radioautographic study on RNA synthesis in aging mouse spleen was conducted after 3H-uridine labeling in vitro. The localization of radiolabelled precursor was used to determine the site of RNA synthesis. The site of the radiolabelled uridine uptake was localized in the haematopoietic cells, particularly in the lymphoblasts. In the labelled cells, most of the silver grains were localized in the nucleus, specifically in the euchromatin. Few cytoplasmic organelles such as the mitochondria and endoplasmic reticulum were labelled with 3H-uridine. Silver grains were also observed over the nucleoli. The labeling index was expressed as the percentage of labelled cells over the total number of cells counted. The labeling index increased from day one after birth and progressively until the 14th day. Thereafter, the labeling index decreased gradually until the 10th month. A significant difference of p less than 0.05 was noted. In all the EMRAG analyzed, it was observed that the number of silver grains per cell increased proportionally with the labeling index. The result of the quantitation of the changes in RNA synthesis correlated well with the maturational development/aging of the animal.