Tritiated water and some tritiated organic compounds (leucine, glucose and thymidine) were administered to rats by oral ingestion and the content of organically bound tritium (OBT) in subcellular fractions (cold PCA soluble, ethanol-ether soluble, hot PCA soluble and alkali soluble) of the liver were determined at various time points after ingestion. In the case of tritiated water, the initial OBT content was high in the cold PCA soluble fraction, which contains low molecular weight components, but as the time proceed the OBT was distributed to other fractions, which contains relatively high molecular weight components. Significant time variation in the OBT content was observed in the hot PCA soluble fraction containing nucleic acids, in which the OBT content, expressed as percentage of OBT content in all fractions, changed from 1 % at 12 hours to 15 % at 50 days. In the cases of tritiated organic compounds, the subcellular distribution of OBT was widely changed owing to their biochemical and metabolic characteristics. Thus, the OBT distribution among subcellular fractions was changed depending on the chemical form at ingestion and on the time after ingestion. The OBT distribution among four subcellular fractions after 22 day' continuous ingestion was also dependent on the chemical form of ingested tritium. Present results should be taken into account for internal dose estimation of tritium in different chemical forms.