Inducible nitric oxide synthase (iNOS) is modulated at the transcriptional level. Overexpression of this protein may result in high levels of nitric oxide leading to tissue damage and immunosuppression. In order to reduce the pathological effects of NO overproduction many efforts have been devoted to the identification of specific inhibitors of iNOS. The discovery of peptide nucleic acids (PNA), a novel class of molecules able to selectively interact with nucleic acids, prompted us to attempt a new way for the regulation of NO production. Here we describe the synthesis, characterization and in vitro effects of a PNA molecule bearing a homopyrimidine sequence complementary to the 5' coding region of murine iNOS mRNA. This PNA shows specific interactions with iNOS mRNA in RNase protection assays and is able to block the synthesis of iNOS protein selectively in a rabbit reticulocyte lysate system. These results strengthen the view of a possible pharmacological application of PNA as a compound able to interfere with a specific enzymatic activity even at low concentrations.