Early experiences can modify regulatory factors affecting gene expression in such a way that, although the DNA sequence itself is not changed, the individual's physiology and behavior is substantially influenced. In some instances these epigenetic effects are exerted upon exposure, while in other instances they are transmitted across generations via incorporation into the germline. Examples of both types of epigenetic effects are presented. First, experience with siblings (littermates) organizes behaviors and their underlying neural substrates in such a way that, as adults, rats and knockout mice behave differently. Second, exposure to the fungicide vinclozolin early in pregnancy imprints the male lineage in such a manner that rats exhibit distinct behavioral profiles as well as unique patterns of gene expression in relevant brain regions. Taken together, this work demonstrates that present and past environments alike modify both social and affiliative related behaviors and their related metabolic activity in specific brain nuclei as well as influencing the abundance of specific genes altering the epigenome in the target brain areas.