In a wide range of organisms, double-stranded RNA triggers posttranscriptional gene silencing or RNA interference (RNAi). Small interfering RNAs, the 21-nt double-stranded RNA intermediates of this natural pathway, have became a powerful tool to knock down specific gene expression in mammalian cell lines and potentially will be useful for the analysis of loss-of-function phenotypes. In mammalian primary neuronal cultures, where genetic manipulations are especially difficult, RNAi might be developed into a highly efficacious tool to study the roles of specific genes in neuron development and functioning. Neurons, however, have been considered the most resistant to RNAi. We report here an application of RNAi to postmitotic primary neuronal cultures. Synthetic siRNA can be readily introduced into neurons and effectively inhibit the expression of endogenous and transfected genes.