This invention relates to antisense oligonucleotides that target mRNAs in cells as substrates for the cellular enzyme RNase H and thereby cause specific degradation of the targeted mRNA. The oligonucleotides have three components: an RNase H activating region, a complementary region and 3 and 5 ends. The invention optimizes each of the components to resist intracellular nucleases, to increase hybridization to target mRNA, to specifically inactivate target mRNA in cells, and to decrease cytotoxicity.