UV-A produced a dose-dependent linear increase of lipid peroxidation in liposomal membrane, as detected by the assay of (i) conjugated dienes, (ii) lipid hydroperoxides, (iii) malondialdehydes (MDA), and (iv) the fluorescent adducts formed by the reaction of MDA with glycine and also a linear dose-dependent increase of [14C]glucose efflux from the liposomes. UV-A-induced MDA production could not be inhibited by any significant degree by sodium formate, dimethyl sulfoxide, EDTA, or superoxide dismutase but was very significantly inhibited by butylated hydroxytoluene, alpha-tocopherol, sodium azide, L-histidine, dimethylfuran, and beta-carotene. MDA formation increased with an increase in the D2O content in water, leading to a maximal amount of nearly 50% enhancement of lipid peroxidation in 100% D2O vis-à-vis water used as dispersion medium. The experimental findings indicate the involvement of singlet oxygen as the initiator of the UV-A-induced lipid peroxidation.