In order to examine the effects of infrared radiation on intraosseous blood circulation, the intraosseous blood flow and oxygen tension were examined in rat tibiae. Infrared radiation was performed for 5 minutes over the ankle joint approximately 40 cm from the skin surface. The intraosseous blood flow was measured with a hydrogen washout technique. Before and after infrared radiation, the blood flow rates in the proximal metaphysis of the rat tibia were 8.7 +/- 2.5 ml/min/100g and 15.6 +/- 5.4 ml/min/100g, respectively, i.e. the intraosseous blood flow increased approximately 80% with the radiation. The intraosseous oxygen tension and temperature were measured with a Po2 sensor, altered to our own specifications. The former increased from 20.4 +/- 6.2 mmHg to 23.3 +/- 6.7 mmHg (about 15%), and the latter from 29.0 +/- 1.4 degrees C to 31.2 +/- 1.9 degrees C, respectively, during the 5 minutes infrared radiation. The results of the present study show that infrared radiation remarkably improves intraosseous blood circulation, and these effects continue for at least 30 minutes. It is, therefore, suggested that infrared radiation can be used as a measure of thermotherapy in some bones to improve intraosseous blood circulation.