Novel reagents for the fluorescent labeling of oligo-and polynucleotides have been prepared: 5-(1-pyrenylethynyl)-2′-deoxyuridine 3′-phosphoramidite and a solid support carrying this nucleoside. Oligonucleotides containing one or several modified units have been synthesized, and the fluorescence of these probes has been shown to change upon hybridization with the complementary sequence.