Summary This study investigated the physiological effects of a thylakoid-bound galactolipase activity, which may participate in thylakoid catabolism during foliar senescence. Electron transport through photosystem II was completely inhibited in thylakoids after only 30 sec of «galactolipase»activation. The thylakoids were completely uncoupled after 60sec. PS I was less sensitive, being reduced by about 60% in high light at the endpoint of «galactolipase» activity. However, light response curves for PS I revealed that the quantum efficiency was very sensitive to galactolipid hydrolysis, although normal rates of electron transport could be achieved by increasing intensities of light. The effects of galactolipid hydrolysis on electron transport were almost entirely eliminated in the presence of BSA, indicating that most, if not all of the disruption was attributable to the accumulation of free fatty acids in the membranes. Ultrastructural examination revealed that hydrolysis of thylakoid MGDG caused dissociation and degeneration of stromal lamellae, while granal stacks remained intact but greatly swollen. The effects were only slightly lessened in the presence of BSA. In contrast, thylakoids exposed to exogenous LNA were largely unstacked, but the lamellae retained their integrity and were more or less paired. Effects of LNA were completely mitigated in the presence of BSA. The structures resulting from lipid hydrolysis resemble those reported in naturally senescing leaves. The results did not indicate that galactolipid depletion adversely affected the photosynthetic function of the thylakoid membranes, but it may contribute to the morphological changes observed in thylakoids during senescence.