Thoron exhalation rates were measured with a newly made portable instrument at 33 areas in 7 prefectures of Japan. Thoron exhalation rates ranged from 49 to 4890 mBq m(-2) s(-1). Radon exhalation rates were also measured in many of the areas at the same time and ranged from 2.1 to 11 mBq m(-2) s(-1). Thoron exhalation rates showed a rough correlation with radon exhalation rates. Both exhalation rates also showed a rough correlation with geological features.