Conjugates of pyrene and perylene with oligodeoxynucleotides were synthesized and tested as hybridisation probes. A 13-mer containing a 3-peryleneacetic acid residue attached to the 5' end through a hexamethylenediamine linker showed no response in the fluorescent spectrum upon hybridisation to the complementary sequence. At the same time, pyrene-labelled probes are sensitive to duplex formation. A pyrene pseudonucleotide unit based on 4-(1-pyrenyl)-1,3-butanediol can be introduced into any predetermined position(s) of the oligonucleotide chain. The probes polylabelled in this fashion displayed considerable changes in the excimer-to-monomer fluorescence intensity ratio after duplex formation. The internal location of two pyrene residues in the probe provides a drastic enhancement of excimer fluorescence (approximately 470 nm) upon hybridisation. When two pyrene units were brought into close proximity to two pyrenes in the complementary strand upon duplex formation, strong excimer emission at approximately 450 nm was detected. This effect provides a basis for a sensor construction designed to detect nucleic acid hybridisation.