Current year needles from 5 yr-old Norway spruce trees, which had been exposed to either episodes of atmospheric O3, or periodic mistings with simulated acid rainwater throughout three summer periods, were-analyzed for changes in molar percentages and ratios of fatty acids isolated from different lipids at the time of maximum winter hardening. No significant changes due to acidic misting were detected but significant decreases in the degree of unsaturation off both C 16 and C18, fatty acids, the molar percentage of δ5,9,12,15, and the molar ratio δ5,918: 2 to δ9,1218:2 in monogalactosyl diglyceride (MGDG) due to summer 03 exposures were found. Molar percentages and ratios of fatty acids did not change much in other lipids bur these changes in plastidie MGDG could be traced to a significant effect of summer O3 on the δ4- and δ12 -desaturases acting upon phosphatidyl choline (PC) in the endoplasmic reticulum. The replacement of the δ6-subset of C18 fatty acids by an equivalent δ5-series throughout was confirmed by Gas chromatography and mass spectrometry Molecular modelling also showed that the δ5 forms, which resembled the δ5-isomers, are very different in shape to the δ5-series and this may account, in part, for the extremely low winter temperatures from which Norway spruce needles may recover.