Phosphorothioate oligonucleotides have a potential as therapeutic agents. The pharmacokinetics, tissue distribution, stability, and cellular uptake by LOX ascites tumor of p120 antisense phosphorothioate oligonucleotide, ISIS 3466, were studied in vivo. The oligonucleotide, which was quickly cleared from the circulation in the normal mice after IV injection, was readily absorbed into the systemic circulation from the peritoneum. The oligonucleotide was found in most tissues 48 h after IP administration. The highest concentrations were in kidney and liver, but the brain had a very low concentration. The phosphorothioate oligonucleotide was intact even after 48 h. When the oligonucleotide was complexed with cationic lipid DOTMA, the DOTMA did not affect the oligonucleotide uptake or tissue distribution in normal mice. However, DOTMA significantly increased the oligonucleotide cellular uptake (4-10 times) in LOX ascites tumors in an IP/IP model. These results indicate that the phosphorothioate oligonucleotide is stable, has favourable kinetics for use as an therapeutic agent, and that DOTMA could be useful in local delivery of the oligonucleotide in vivo.