The intratissue pH of SCK tumor and leg muscle of unanesthetized mice were determined before, during and after hyperthermia with the use of bulb type pH microelectrodes having pH-sensitive hemisphere 20-40 micron in outer radius. Intratumor pH was heterogeneous throughout tumor (range, 6.60-7.38; average, 6.96), and was more acidic than the intramuscle pH of mouse leg (average, 7.46). Hyperthermia at 43.5 degrees C for 30 min induced a further increase in acidity (decrease in pH of about 0.2 units) in tumor but not in muscle. The heat-induced acidity in tumor lasted for 12 hr following hyperthermia and then recovered to almost control pH value 24 hr after heating. The cause of the increase in acidity in the heated tumors is not clear, but it appears to result from an increase in the contents of acidic metabolites and a sluggish drainage of them due to induced vascular damage. The increased acidity in the heated tumors may inhibit the repair of thermal damage and sensitizes the tumor cells to subsequent heating.