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 (~ 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 ~ 450 nm was detected. This effect provides a basis for a sensor construction designed to detect nucleic acid hybridisation.