Weak red light-induced changes in chlorophyll fluorescence parameters and in the distribution of PS I and PS II in thylakoid membranes were measured in wheat leaves to investigate effective ways to alter the excitation energy distribution between the two photosystems during state transition in vivo. Both the chlorophyll fluorescence parameter Fm/Fo and F685/F735, the ratio of fluorescence yields of the two photosystems at low temperature (77 K), decreased when wheat leaves were illuminated by weak red light of 640 nm, however, Fm/Fo decreased to its minimum in a shorter time than F685/F735. When Photosystem (PS II) thylakoid (BBY) membranes from adequately dark-adapted leaves (control) and from red light-illuminated leaves were subjected to SDS-polyacrylamide gel electrophoresis under mildly denaturing conditions, PS I was almost absent in the control, but was present in the membranes from the leaves preilluminated with the weak red light. In consonance with this result, the content of Cu, measured by means of the energy dispersive X-ray microanalysis (EDX), increased in the central region, but decreased in the margin of the grana stacks from the leaves preilluminated by the red light as compared with the control. It is therefore suggested that: (i) both spillover and absorption cross-section changes are effective ways to alter the excitation energy distribution between the two photosystems during state transitions in vivo, and the change in spillover has a quicker response to the unbalanced light absorption of the two photosystems than the change in light absorption cross-section, and (ii) the migration of PS I towards the central region of grana stack during the transition to state 2 leads to the enhancement of excitation energy spillover from PS II to PS I.