The purpose of the present study is to develop an accurate and practicable method to estimate an internal dose after exposure to tritium in various chemical forms. In rats exposed to tritiated water (HTO) or some tritiated organic compounds by single ingestion, the concentrations of total tritium and organically bound tritium (OBT) in blood and various organs were determined at various time intervals after ingestion. The concentrations of total tritium in blood showed a tendency to be higher than those in the majority of organs. When the cumulative doses to blood and organs for 100 days after ingestion of various tritiated compounds were compared, the doses to blood were almost the same or higher as compared with the maximum doses to organs. These results indicated that blood analyses would be useful to estimate a maximum of internal doses for exposure to tritium in various chemical forms. It was also suggested that the concentration ratio of OBT to total tritium in blood could be used to deduce the chemical form of tritium at exposure and the elapsed time after exposure.