The properties of microsomal membranes in spring wheat leaves (Triticum aestivum L. cv. Ganlong No.92-005) exposed to (0) control, 22 21 8.64 (T) and 11.2kJ m day (T) biologically effective UV-B irradiation (UV-B ) were studied under greenhouse conditions. These 1 2 BE irradiance levels correspond to a decrease in the stratospheric ozone of ø12.5 and 20%, respectively, for a clear solstice day at Lanzhou (36.048N, 1550 m), China. Compared with controls, the content of malondialdehyde (MDA) increased by 70.8% in T and 83.8% in Ton 1 2 the 7th day of the radiation, and the IUFA (index of unsaturated fatty acids) decreased, indicating peroxidation of lipid acids. Simultaneously, a drastic decrease of phospholipid content after 21 days and an increase of membrane lipid microviscosity on UV-B irradiation were also found, suggesting a reduction in the fluidity of membrane lipids. Ethylene emission by the microsomal membrane, in the presence of exogenous 1-aminocyclopropane-1-carboxylic acid was higher in the wheat seedlings after 7, 14 and 21 days' irradiation 21 than in the controls. These changes were correlated with a rise in lipoxygenase activity. Membrane-bound enzymes (Ca-ATPase and 21 Mg-ATPase) were promoted by UV radiation in the first 7 days and significantly decreased after 14 and 21 days' treatment in comparison to control. Our results suggest that UV-B radiation may cause changes in structural complexity and function of microsomal membranes in spring wheat leaves.