Headgroup and acyl chain NBD-labeled phospholipid derivatives, viz. 1,2-dipalmitoyl-sn-glycero-3-phospho-[N-(4-nitrobenz-2-oxa-1,3-diazole)-ethanolamine] (DPPN) and 1-acyl-2-[12-[(7-nitro-2,1,3-benzoxadiazol-4-yl)amino]dodecanoyl]-sn-glycero-3-phosphocholine (NBD-PC), were employed to study lipid conformations and organization in the course of the main phase transition of dipalmitoylglycerophosphocholine (DPPQ liposomes. The differential sensitivity of the 335 and 470 nm absorption bands to environment polarity was used to explore the ground-state NBD environment. Anisotropy measurements with excitation at those bands revealed the presence of two NBD-PC populations below T-m. The fluorescence maximum emission intensity, the ratio of emission intensities with excitation at 470 and 335 run, and the rate coefficient k(1) for the reduction of NBD by dithionite to nonfluorescent derivatives undergo complex changes in the course of the main phase transition of DPPC. For DPPN, the alterations in k(1) and intensity ratio are likely to originate from a redistribution of this fluorophore at the transition and suggest a maximum in the bilayer transverse compressibility. For NBD-PC, the changes in these properties appear to be related to a shift in the fluorophore distribution at the percolation threshold for the two-phase region and self-quenching of the NBD moiety.