Antisense oligodeoxynucleotides (ODN) that disrupt gene function, leading eventually to a reduction in levels of the encoded protein, are powerful molecular tools for studying the functions of neuropeptides and their receptors in the brain. Their potential usefulness is even greater for the ever increasing number of newly discovered peptide and receptor genes expressed in the central nervous system for which pharmacological blocking agents or selective ligands may not be available. Generally, antisense ODNs are short DNA sequences complementary to a specific mRNA that has the same sequence as the sense DNA strand. Although the mechanisms of antisense action are not fully understood, it prevents translation of the mRNA into protein by action at any one of several sites in the sequence of events from DNA to protein synthesis. This inhibition may occur (1) by binding to DNA to form a triple-helical structure; (2) by simply hybridizing to the target RNA and thus preventing its interaction with ribosomes, polymerases, and so forth; and (3) by binding to mRNA to provide a substrate for enzymes such as RNase-H that cleave and degrade the mRNA and prevent protein expression. Additionally, there is evidence that antisense ODNs may bind directly to the target proteins, resulting in its inhibition. The latter effect (referred to as aptomer binding) occurs in a non-antisense manner, however, inhibition of a specific protein may be useful from a therapeutic point of view. Whatever may be the mechanism of antisense ODN action, it is a powerful alternative approach to assess protein function, especially in the absence of pharmacological blockers.