Mice deficient for the cellular prion protein (PrP^C) do not develop prion disease; accordingly, gene-based strategies to diminish PrP^C expression are of interest. We synthesized a series of chemically modified antisense oligonucleotides (ASOs) targeted against mouse Prnp messenger RNA (mRNA) and identified those that were most effective in decreasing PrP^C expression. Those ASOs were also evaluated in scrapie-infected cultured cells (ScN2a) for their efficacy in diminishing the levels of the disease-causing prion protein (PrP^Sc). When the optimal ASO was infused intracerebrally into FVB mice over a 14-day period beginning 1 day after infection with the Rocky Mountain Laboratory (RML) strain of mouse prions, a prolongation of the incubation period of almost 2 months was observed. Whether ASOs can be used to develop an effective therapy for patients dying of Creutzfeldt–Jakob disease remains to be established.