Pea (Písum sativum L.) leaves fully developed under either high or low irradiance were exposed for up to 30 h to dark oř limiting, saturating oř excessive iřřadiance. The time courses of degradation of a popidation of radioactively prelabelled Dl protein and the fluorescence parameter Fv/Fm, measured on intact leatf discs at room temperature were studied. The Dl protein degradation was shown to be dependent on irradiance but not directly correlated with changes in photosynthetic capacity. The irradiance dependence of Dl protein degradation was different in the two types of leaves and it occurred at a slow but significant rate in the daik. The irradiance dependence of Dl degradation tnay be associated with the 3-dimensional structure of the thylakoid membrane and is suggested to play a key role in flie regulation of radiant energy absorption and photosynthetic capacity of flie leaves.