A new polarographic histograph was tested in tumors to assess its potential adverse effects in mice. The tumor cell lines used were: the lewis lung carcinoma (3LL) and two human xenografts (Na11+ and HRT18). The C57BL/6 mice survival and number of pulmonary metastases were not altered after pO2 measurements. Whatever the cell line, tumor doubling times were not changed after pO2 measurements. The new polarographic histograph was found to be reliable.