The mono(pyridine-N-oxide) analog of the H4dota macrocylic ligand, H3do3a-pyNO, is capable of forming thermodynamically stable and kinetically inert Ln(III) complexes. Its Eu(III) and Tb(III) complexes display a strong long-lived fluorescence as a result of the antenna effect of the pyridine- N-oxide fluorophore in the reagent. It is shown that H3do3a-pyNO can be used as a fluorogenic reagent for the determination of Eu(III) and Tb(III) at pH 6.5 and cL 1 mM. At an excitation wavelength of 286 nm, the emission maxima are 615 nm (Eu(III)-complex), and 547 nm (Tb(III)complex). Detection limits are at concentrations around 1.0 mM and linearity of the method spans over 2 orders of magnitude. The method was applied to artificial and real samples (spiked mineral waters, extracts from cathode ray tube luminophore dust) and gave satisfactory results. The method is simple, rapid, and hardly interfered by other metal ions.