On experimental rat tumors, preferentially on DS-Carcinosarcoma, special glass electrodes were used to measure and to register development of tissue-pH after injection of glucose into the rats. On tumor surface pH-values were lowered more quickly and increased much faster as in tumor depth. The lowest registered pH was about 5,9. High s. c. dosages of Insulin caused a marked decrease of pH-values for nearly one hour too. In metabole effective small tumors tumor acidity could be regulated satisfactory with administered glucose and insulin between pH 7,0 and 6,0. Determination of lactate concentration in tumor tissue, surrounding the electrode, indicated a linear correlation between pH-value und lactate concentration in the range between pH 6,0 and 7,0. Regression coefficient comes to — 40,9 ± 2,2 ×10-3 mMol/g wet weight and pH-unit. It is emphasized that according to the varying cell supply the compact tumor shows no homogeneity on pH-rates, acidification kinetics, and sensibility against acidotic cell lesion. Results are criticized with reference to other publications and their significance for the theory of tumor lesion by alkylating agents is discussed.