The fluorescence power from biological tissue excited by a femtosecond laser pulse compared with excitation power does not appear to obey a simple quadratic relationship given by the steady non-linear theory. A more reliable analysis is developed based on transient two-photon absorption because the response time of two-photon absorption is longer than the width of a femtosecond pulse. Good agreement is obtained between the theoretical analysis and the experimental results of fluorescence power versus excitation power. This letter offers potential value to non-linear optics in biological tissues.