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Fluorescence excitation spectra are recorded from HpD in living RUC -2 tumors implanted in rats.The experiments show that, compared with the excitation spectrum of HpD in solution, the Soret excitation band at 400 nm decreases with respect to the minor excitation bands at longer wavelengths. The excitation efficiencies at 503 nm and 625 nm are comparable with the efficiency at 400 nm. The differences in the in vivo and in vitro excitation spectra are caused by the wavelength dependent penetration depth of the excitation light due to absorption of blood and tissue.