The flat heterocyclic antitumoral drug pazelliptine (PZE) strongly interacts with nucleic acids. The transient absorption spectra of PZE/polynucleotide complexes have been investigated with subpicosecond time resolution on a large spectral range, 400-850 nm. The relaxation dynamics are found to vary widely depending upon the type of polynucleotide in which PZE has been inserted. For fully protonated PZE and for PZE complexed to poly(dA-dT)-poly(dA-dT), a similar fast spectral relaxation was detected and assigned to a deprotonation of the excited singlet state of the drug. In the case of the PZE/poly(dG-dC)-poly(dG-dC) complex, the evolution of the transient absorption spectra suggests that there are at least two different drug binding sites. The kinetics of the differential absorption reveals the activation of a radiationless transition S-1-->S-0 upon PZE association to poly(dG-dC)-poly(dG-dC). The possible mechanism of this fast molecular process is discussed in connection with previously reported results on heterocycle-nucleotide interactions.