We have investigated the intracellular fate and antisense effect of oligonucleotide/cationic liposome complexes using phosphorothioate oligonucleotides (S-Oligo) targeted to inducible nitric oxide synthase in mouse peritoneal macrophages. Confocal laser microscopic analysis revealed that, after application of fluorescein isothiocyanate (FITC)-labeled S-Oligo alone, the intracellular localization of fluorescence exhibited a punctate pattern in the cytoplasm, suggesting that the oligonucleotides were mainly confined to the endosomal and/or lysosomal compartments. In the case of complexation with Lipofectin and DMRIE-C liposomes, cellular uptake of FITC-S-Oligo was not greatly enhanced and the fluorescence localization in the cells was similar to that of FITC-S-Oligo alone. LipofectAMINE slightly enhanced cellular uptake of FITC-S-Oligo: however, the intracellular localization profile of FITC-S-Oligo remained largely unchanged. The antisense effect was slightly enhanced by LipofectAMINE under only very limited experimental conditions. It was concluded that cationic liposomes are not a potential carrier for S-OLigo in peritoneal macrophages because of their inability to promote the release of S-Oligo from the endosomal compartments to the cytosol over a non-toxic concentration range.