Benzoporphyrin-derivative (BPD)–monoacid-ring A photodynamic therapy (PDT) was performed on subcutaneous tumor implants in a rat ovarian cancer model. In order to assess PDT efficacy the tumor and normal tissue optical properties were measured noninvasively prior to and during PDT using frequency-domain photon migration (FDPM). FDPM data were used to quantify tissue absorption and reduced scattering properties (given by the parameters μ_a and μ′_s, respectively) at four near-infrared (NIR) wavelengths (674, 811, 849 and 956 nm). Tissue physiologic properties, including the in vivo concentration of BPD, deoxy-hemoglobin (Hb), oxy-hemoglobin (HbO₂), total hemoglobin (TotHb), water (H₂O) and percent tissue hemoglobin oxygen saturation (%S_tO₂), were calculated from optical property data. PDT efficacy was also determined from morphometric analysis of tumor necrosis in histologic specimens. All the measured tumor properties changed significantly during PDT. [Hb] increased by 9%, while [HbO₂], [TotHb] and %S_tO₂ decreased by 18, 7 and 12%, respectively. Using histologic data we show that long-term PDT efficacy is highly correlated to mean BPD concentration in tumor and PDT-induced acute changes in [HbO₂], [TotHb] and %S_tO₂ (correlation coefficients of 0.829, 0.817 and 0.953, respectively). Overall, our results indicate that NIR FDPM spectroscopy is able to quantify noninvasively and dynamically the PDT-induced physiological effects in vivo that are highly correlated with therapeutic efficacy.