Radon decay product activity was measured in saliva of 10 male patients 20-30 min after a 1-hour radon exposure in the gallery of the Gasteiner Heilstollen (radon activity 36.2 kBq/m(3), radon progeny activity 20.3 kBq/m(3)), in 1 patient showing relatively high activity (75th percentile) measurements were continued until 65 min after exposure. Patients were asked to collect about 2 mi of saliva in the mouth and produce it on a filter. After drying the filter at 300 degrees C, radon progeny activity was measured. Activity (median) at 20-30 min after leaving the treatment area was 4.5 Bq (25th percentile 1 Bq; 75th percentile 21 Bq). In the patient who underwent additional measurements the activity showed a further increase up to 29 Bq (35 min after radon exposure) before it continuously decreased to a very low activity (1-3 Bq) at 65 min after exposure. The results show that a significantly increased radon decay product activity is found in saliva after speleotherapeutic radon exposure. Maximum values were observed 35 min after radon exposure. Radon decay product activity almost disappeared after about 1 h.