The purpose of this study is to investigate the biokinetics of inhaled radon, radon activity concentrations in mouse tissues and organs were determined after mice had been exposed to about 1 MBq/m(3) of radon in air. Radon activity concentrations in mouse blood and in other tissues and organs were measured with a liquid scintillation counter and with a well-type HP Ge detector, respectively. Radon activity concentration in mouse blood was 0.410 +/- 0.016 Bq/g when saturated with 1 MBq/m(3) of radon activity concentration in air. In addition, average partition coefficients obtained were 0.74 +/- 0.19 for liver, 0.46 +/- 0.13 for muscle, 9.09 +/- 0.49 for adipose tissue, and 0.22 +/- 0.04 for other organs. With these results, a value of 0.414 for the blood-to-air partition coefficient was calculated by means of our physiologically based pharmacokinetic model. The time variation of radon activity concentration in mouse blood during exposure to radon was also calculated. All results are compared in detail with those found in the literature.