The emission of modulated 685-nm chlorophyll fluorescence from intact leaves was measured using a pulsed (yellow) exciting light of very low intensity combined with various non-modulated lights of sub- and supersaturating intensities. It was found that, whereas the “extreme” (initial and maximal) fluorescence levels were drastically quenched by high light intensities, the level of steady-state modulated fluorescence in light-adapted leaves was virtually constant and independent of the intensity and the quality of the continuous light used to drive photosynthesis. These results provided an example of biological homeostasis. For instance, the modulated fluorescence emission from a dark-adapted leaf with all photosystem II reaction centers in the open configuration was similar to the steady-state modulated fluorescence emission measured in leaves illuminated with an additional strong, photosynthetically saturating light with all photosystem II traps closed. This homeostatic behavior was characteristic of only healthy and mature leaves. It was strongly perturbed in young leaves and also in leaves exposed to unfavorable environmental conditions. In some stress situations (e.g. leaf dehydration), noticeable deviations from homeostasis were detected before any significant changes in the familiar photochemical (q_p) and non-photochemical (q_NP) fluorescence quenching coefficients could be observed, suggesting that the analysis of this “bio-energetic homeostasis”, revealed by the examination of raw fluorescence data, could be a useful approach for early detection of subtle changes in the physiological state of plants.