The fluorescence intensity of diphenylhexatriene (DPH) and of trimethylammonium-diphenylhexatriene (TMA-DPH) is measured when these probes are embedded in vesicles of dipalmitoyl- and dioleoylphosphatidylcholine (DPPC and DOPC), in mixtures of these vesicles as well as in vesicles of the mixed phospholipids, in trout intestinal brush border membranes and in mitoplasts of rat liver cells. The intensity in DOPC vesicles is found to be significantly higher than in DPPC vesicles. When these systems are irradiated with strong ultraviolet light radiation, a decrease in the fluorescence intensity is observed; this effect is much stronger in DOPC than in DPPC vesicles. The fluorescence anisotropy values in the mixture of vesicles as well as in the membranes show an initial increase with irradiation which is followed by a significant decrease. A transfer of DPH molecules between DPPC and DOPC vesicles is observed. For TMA-DPH this transfer takes place only from DPPC to DOPC vesicles, but not vice-versa. These results are related to intensity and anisotropy measurements of these probes in cell cultures.