Antisense oligonucleotides can inhibit gene expression in living cells by binding to complementary sequences of DNA, RNA or mRNA. The mechanisms include inhibition of RNA synthesis, RNA splicing, mRNA export, binding of initiation factors, assembly of ribosome subunits and of sliding of the ribosome along the mRNA coding sequence. The most efficient antisense oligonucleotides also activate RNAse H, an ubiquitous enzyme that cleaves the mRNA at sites of mRNA/oligonucleotide duplex formation. A staggering number of oligonucleotide modifications have been proposed to retard degradation by nucleases, enhance cellular uptake, increase binding to the target sequence, and minimize non-specific binding to related nucleic acid sequences. Phosphorothioates are the most popular oligonucleotides used in cell culture and in vivo, although sequence non-specificity remains an underreported problem. Recently developed chimeras between methylphosphonates and phosphodiester oligonucleotides appear to combine the advantages of water solubility, nuclease resistance, enhanced cellular uptake, activation of RNAse H, and high sequence selectivity. Antigene oligonucleotides are also promising, because they can inhibit gene expression by triple helix formation with DNA or by binding to one of the DNA strands. They have so far been little used in physiological studies. Cost is still a prohibitive factor, especially for suppressing the expression of a hormone or hormone receptor gene in rats, for example. However, patch-clamp dialysis of single cells or nuclear microinjections in culture, exposure of cultures to extracellular oligonucleotides, and intra-cerebral microinjections of oligonucleotides are feasible and highly rewarding approaches in physiology.