The character of structural changes in thylakoid membranes caused by temperature variation was investigated. Experiments were performed on maize leaf segments in vivo and in a closed temperature cycle 24-50-24 degrees C. Two biophysical methods were used for detection: luminescence and EPR. Arrhenius plots of delayed fluorescence (DF) versus reciprocal temperature revealed two break points, T1 and T2. The MeFASL (10.3) spin probe monitoring properties close to membrane surface detected only T2 transition temperature. The results were interpreted in terms of a fluidity change which starts in a membrane centre at T1 and gradually displaces toward the surface at T2. The T1 and T2 transition temperatures are sensitive to pretreatment history of plants indicating that high temperature and drought-induced membrane alterations are irreversible. Activation energies E1, E2 and E3 were determined for temperature regions below T1 between T1 and T2, and above T2, respectively. The E1 and E3 activation energies showed greater sensitivity to stress than did E2. There are some indications that the DF method could be used to screen temperature sensitive and temperature resistant genotypes.