In this study we investigated how polarographically measured pOz profiles of experimental tumors were affected by histological type, site of tumor implantation, and the use of two different anesthetic agents, sodium pentobarbital and ketamine with xylazine. Oxygen measurements were obtained in four syngeneic murine tumors, the sarcomas Sa-NH and FSa-II, the mammary carcinoma MCa-4, and the ovarian carcinoma OCa-I, and in normal muscle. Comparison of the pOz measurements revealed significant differences between various groups in both the percentage of p 0 2 measurements less than or equal to 2.5 mm Hg, and the median PO?. OCa-I was significantly more hypoxic than the other tumors. Three of the four tumors growing subcutaneously were numerically less well oxygenated than their intramuscular counterparts, significantly so in the case of MCa-4. Both types of anesthesia significantly reduced measured pol for intramuscular Sa-NH and MCa-4. It is essential that variables such as tumor type, tumor site, and anesthesia be controlled for when interpreting the radiobiological significance of pOz measurements and when comparing results obtained in different laboratories. Furthermore, these observations imply the need for strict standardization of clinical protocols in order for PO! measurements to have any real prospect as a predictive assay.