Environmental enrichment (EE) is a largely employed behavioral procedure in which animals are exposed to high stimulation compared with conventional housing conditions. Animal exposure to an EE exerts beneficial effects on the performance of different learning tasks and induces a number of behavioral, neurochemical, and neuroanatomical changes including hippocampal cell proliferation. However, the importance of voluntary interaction with the environment in these changes has not been clearly resolved yet. Moreover, the effects of a complex environment on animal emotionality still remains questionable and has not been explored in detail under conditions that allow unmasking individual responses among subjects in a group. The present study was aimed at exposing groups of rats to an EE, and analyzing individual differences in activity levels during EE sessions. We observed differences with respect to the activity level displayed by rats during the enriched sessions, which correlated with differences in the rate of hippocampal cell proliferation. It is suggested that exposure to EE may reduce anxiety-like behaviors and may elicit individual differences on emotional reactions positively linked with hippocampal neurogenesis and testosterone levels. © 2007 Wiley Periodicals, Inc. Develop Neurobiol, 2007.