The pyridazinone BAS 13-338 (SAN 9785) inhibits the desaturation sequence leading to polyunsaturated fatty acids, mainly of glycolipids. Parallel to the inhibition of fatty acid desaturation in the presence of the pyridazinone, changes in energy-distribution parameters have been observed. These data indicate that the amount of polyunsaturated fatty acids in glycolipids is strongly correlated with excitation, trapping, grouping and dissipation, but not with spillover. Functional changes in energy distribution induced by BAS 13-338 are interpreted as a consequence of structural changes in the lipid matrix, which may imply a structure-function relationship between pigment protein complexes and the surrounding lipid environment in thylakoids.