Oxic and Hypoxic Cells in a Murine Squamous Cell Carcinoma

[(125)I]Iododeoxyuridine labeling of a squamous cell carcinoma and follow‐up of (125)I activity at the tumor in situ revealed that the (125)I activity remained at a constant level from the 24th to the 100th hour post‐labeling and then decreased with a half time of about 200 hr. Autoradiographic studies with [(3)H]thymidine showed that the tumor cells were labeled around capillaries, spread through the corded structure (the cord) and finally reached the necrotic regions. One could speculate that the constant (125)I period represents the transit time of the labeled cells through the cord and that the decline occurs mostly in the necrotic regions. X‐Irradiation shortened the constant period of (125)I activity by about 24 hr and accelerated the declining rate in a dose‐dependent manner. When tumors were made hypoxic by clamping the legs, the declining rate decreased significantly. When misonidazol (a hypoxic radiosensitizer) was administered before X‐rays, the declining rate increased to a level higher than that of the oxic tumors. From the time course studies, it was suggested that the tumor cells immediately after (125)I‐labeling were oxic, that they became gradually hypoxic during their transit through the cord and that they became anoxic when they reached the necrotic regions.