We examined the effects of repeated artificial CO2 (1,000 ppm) bathing on tympanic temperature (T(ty)), cutaneous blood flow, and thermal sensation in six healthy males. Each subject was immersed in CO2 -rich water at a temperature of 34 degrees C up to the level of the diaphragm for 20 min. The CO2 -rich water was prepared using a multi-layered composite hollow-fiber membrane. The CO2 bathing was performed consecutively for 5 days. As a control study, subjects bathed in fresh water at 34 degrees C under the same conditions. T(ty) was significantly lowered during CO2 bathing (P < 0.05). Cutaneous blood flow in the immersed skin (right forearm) was significantly increased during CO2 bathing compared with that during fresh-water bathing (P < 0.05), whereas cutaneous blood flow in the non-immersed skin (chest) was not different between CO2 and fresh-water bathing. Subjects reported a "warm" sensation during the CO2 bathing, whereas they reported a "neutral" sensation during the fresh-water bathing. The effects of the repeated CO2 bathing were not obvious for core temperature and cutaneous blood flow, but the thermal sensation score during the CO2 bathing was reduced sequentially by repeated CO2 bathing (P < 0.05). These thermal effects of CO2 bathing could be ascribed largely to the direct action of CO2 on vascular smooth muscles and to the activity of thermoreceptors in the skin. Serial CO2 bathing may influence the activity of thermoreceptors in the skin.