A simple fluorimetric assay has been developed for probing AT-rich regions in DNA. A fluorescent ligand, 1,4-bis[2-(1-methylpyridinium-4-yl)vinyl] benzene diiodide (pMPVB), binds to GC and AT sequences with comparable affinity and the only fluorescent species is trans-pMPVB/AT complex. Since the ligand undergoes photoisomerization, the system is initially photoequilibrated to reach a photostationary state (pss), which contains a high fraction of the non-fluorescent cis-isomer. Subsequent ''analytical'' irradiation at the wavelength of isosbestic point (357nm) leads to a new pss with higher content of the fluorescent trans-isomer (in situ generation of trans-isomer). An increase in fluorescence, recorded during the analytical irradiation, is considered as an analytical signal that is proportional to the content of A and T bases arranged in continuous sequences of 4bp or more (AT quartets). The calibration graph, plotted for mixtures of homopolymers [poly(dA-dT)] 2 and [poly(dG-dC)] 2 at varied molar ratios, was linear over the range of 0.1-0.7 molar fraction of AT quartets with the detection limit (3σ) of 0.03. The method has been proved experimentally on two natural DNA samples, calf thymus and T4-phage DNA, giving molar fractions of AT quartets of 0.17 and 0.26, respectively, in good agreement with expected results calculated from the published genomic sequences. The relative standard deviation (n=3) was within 7%.