The influence of photodynamic therapy (PDT) on vascular perfusion was investigated in 2 S.C. mouse tumours, a radiationinduced fibrosarcoma (RIF I) and a squamous-cell carcinoma (SCCVll). The 86Rb extraction technique was used to measure changes in perfusion relative to cardiac output at various intervals after interstitial PDT. Control groups showed that vascular perfusion in the RIF I tumours decreased with increasing tumour size. For both tumours, of constant size, vascular perfusion decreased to less than 10% of control values within 5 min after high PDT doses. Significant decreases in vascular perfusion were also seen after lower, sub-curative doses. Thereafter there was slow recovery towards control levels. Photofrin given at shorter intervals before illumination generally resulted in even larger decreases in tumour perfusion, and slower recovery. Comparison of tumour perfusion measurements after PDT with tumour response revealed an inverse correlation with tumour growth delay both for the RlFl and for the SCCVll tumours. PDT with sub-curative light doses appears to decrease vascular perfusion in the RIF I and SCCVll for a period of at least 24 hr. The most severe reductions in turnour blood flow were