Immediate early genes (IEGs) are a class of genes that show rapid and transient but protein synthesis-independent increases in expression to extracellular signals such as growth factors and neurotransmitters. Many IEGs code for transcription factors that have been suggested to govern the growth and differentiation of many cell types by regulating the expression of other genes. IEGs are expressed in adult neurons both constitutively and in response to afferent activity, and it has been suggested that during learning, IEGs may play a role in the signal cascade, resulting in the expression of genes critical for the consolidation of long-term memory. Long-term potentiation (LTP) is a persistent, activity-dependent form of synaptic plasticity that stands as a good candidate for the mechanism of associative memory. A number of IEGs coding for transcription factors have been shown to transiently increase transcription in the dentate gyrus of rats following LTP-inducing afferent stimulation. These include zif/268 (also termed NGFI-A, Krox-24, TIS-8, and egr-l), c-fos-related genes, c-jun, junB, and junD. Of these, zif/268 appears to be the most specifically related to LTP since it is evoked under virtually all LTP-inducing situations and shows a remarkably high correlation with the duration of LTP. There are a number of outstanding questions regarding the role of zif/268 and other IEGs in LTP, including which second messenger systems are important for activating them, which "late effector" genes are regulated by them, and the exact role these genes play, if any, in the stabilization and maintenance of LTP.