The effects of balneological procedures as well as the duration of hospitalization on blood pH, P(CO(2)) and P(O(2)) were studied in fifty-two patients chiefly including rheumatoid arthritics. The changes of these parameters in sixty minutes after any foment were discussed comparing with those in the control groups. The results were as follows. 1) Blood pH decreased in value from first to second hospital week and thereafter remained almost constant. No P(CO(2)) variation was observed. The P(O(2)) remained stable till third hospital week and then followed by up and down changes during the forth to the ninth week. 2) The arterial blood pH, P(CO(2)) and P(O(2)) in all showed a tendency to increase after the general spring bath of ten minutes duration (bath temp. : 40±1℃). The venous blood pH and P(O(2)) also increased as in the arterial but P(CO(2)) decreased. These non significcant changes in the venous blood were considered to be resulted from the increased cutaneous blood flow in the forearm. 3) It was showed that mud bath of twenty minutes duration (bath temp. : 40±1℃) made an influence on the venous blood pH, P(CO(2)) and P(O(2)) via two different mechanisms. The first is the increased blood flow which had five minute value of P(O(2)) and pH elevated but P(CO(2)) reduced significantly. The second is the prominent increase of oxygen uptake in which the significant reduction of five minute P(O(2)) value but no variation of pH or P(CO(2)) was demonstrated. It was suggested that these two types of variations were based on the difference of individual response to heat. 4) The signiilcant blood pH, P(CO(2)) and P(O(2)) changes after mud pack of twenty minutes duration (surface temp. : 40~50℃) were considered to reflect the increased blood flow. 5) The increased cutaneous blood flow induced by balneotherapeutical procedures was proved to remain as long as sixty minutes thereafter in almost all cases. 6) The blood pH, P(CO(2)) and P(O(2)) changes observed in the control groups were related mainly with the increased blood flow and only little, if any, with the elevated oxygen uptake.