Using monochromatic laser excitation, narrowing of the porphyrins 0‐0 fluorescence line in rat muscle tissue to a width of 50‐70 cm−lat 4.2 K is observed. Fluorescence spectra at different excitation wavelengths and excitation spectra are measured. Fluorescence line peak follows the shift of the excitation frequency. Several of the most prominent vibrational frequencies of porphyrins in the excited electronic state are deter