Photodynamic therapy (PDT) with verteporfin provides a reliable way to destroy malignant tissues. Changes in the blood flow and oxygen partial pressure (pO₂) during verteporfin-PDT were studied here in the tumor tissue of the rat mammary R3230Ac carcinoma model. Oxygen microelectrodes (6–12 μm tip diameter) were used to measure the transients locally within tumors during intravenous injection of 1.0 mg/kg verteporfin followed by irradiation 15 min later with 690 nm light at 200 mW/cm², for a cumulative dose of 144 J/cm². The observed changes in pO₂ were heterogeneous and there was a difference in the response of low-pO₂ regions relative to higher-pO₂ regions. The change in pO₂ in hypoxic tissue regions (pO₂ < 8 mmHg) had acute pO₂ loss after treatment, whereas the response in regions of higher pO₂ (>8 mm Hg) was more heterogeneous with some areas maintaining their pO₂ value after treatment was completed. Blood flow measurements taken on a subset of the animals indicated a significant loss in flow during the initial light delivery that remained low after treatment, indicating some vascular stasis. The results suggest that hypoxic or poorly perfused vessels may be more susceptible to acute stasis than normoxic vessels in this treatment protocol.