In hepatocytes of fetal rats, cytoplasmic organelles identifiable as microbodies appeared, although only a few of them showed nucleoids and most of them generally had an electronlucent appearance due to the low density of their matrices. Some of these microbodies, especially those lacking the nucleoid, showed a substantial connection with granular endoplasmic reticulum (ER), suggesting that microbodies might be formed from granular ER. Agranular tubular profiles projecting from the surface of microbodies were found with a high frequency in fetal and neonatal rats; however, this phenomenon may not provide crucial evidence suggestive of the derivation of microbodies from agranular ER. Growth and maturation of microbodies are considered to be brought about by an enlargement of these organelles, an increase in their matrices, an appearance and enlargement of the nucleoids, and an increase in the enzyme involved. The specific activity of urate oxidase in the isolated nucleoid fraction was significantly lower in the earlier stages of postnatal growth than later. Increases in the enzyme activity per nucleoid (maturation of the nucleoid), in the number of microbodies containing nucleoids (formation of the nucleoid), and in the size of nucleoids (growth of the nucleoid), may contribute to increases in the enzyme activity of the tissues.