The aging process affects all body tissues including the brain and different hormones reduced with age in mammals. This research was performed to study the effects of hormonal changes during the aging process on neurogenesis in the hippocampus. The female Wistar rats were divided into five groups including pre-pubertal, pubertal in metestrus phase, pubertal treated with 17β-estradiol, post-pubertal in metestrus phase and post-pubertal treated with 17β-estradiol. The groups received 50 μg/kg 17β-estradiol subcutaneously daily injections for 7 days. Groups in metestrus phase were detected in based on the observed cells in the vaginal smear. The results revealed that cell proliferation along with 17beta-estradiol injection in the CA1 region of hippocampus was increased and cell proliferation in the adult group was significantly higher than the after puberty group. INTRODUCTION Estrogens have well known effects on reproductive behaviors and associated brain regions; however, estrogens also influence non-reproductive behaviors such as cognition and associated brain regions such as the hippocampus. Past research in this area has shown a strong but complex relationship between estrogens and cognition, with many profound alterations in neuroplasticity in the hippocampus coinciding with estrogens’ influence on cognition. The hippocampus is a limbic structure that is critical for spatial, contextual, and relational memory formation (Eichenbaum, 2004) and is involved in and responsive to stress (McEwen et al., 2007). Interestingly, the hippocampus shows a remarkable degree of plasticity in response to steroid hormones such as estrogens and glucocorticoids (Galea et al., 2006; Galea, 2008; Galea et al., 2008). Hormones are changed somewhat during the aging process. Sexual hormones are decreased with age in mammals and the changes in the human appear with having disorders in the nerve cells regeneration, depression and other psychological disturbances (Garcia-Segura, 2009). The wide variety of the neurological disorders will necessitate more research on treatment and since the majority of neurons are involved in neurological disorders, more studies are required regarding the proliferation and persistence issues. Furthermore, Neurogenesis process decreases with age (Kuhn et al., 1996; Kempermann et al., 1997; Nilsson et al., 1999). Estrogen is a sexual hormone involved in the reproductive system, and the nervous system is also affected. Thus, estrogen acts more than a sexual hormone; steroids perform their actions by binding to two intracellular receptors, ERα and ERβ (Sherwin, 2006; Shughrue et al., 2000). Aging affects all body tissues, including the brain and different hormones, including growth hormone, IGF-1 and finally sexual hormones are reduced with aging in mammals. This suggests that changes in hormone levels due to aging may have negative effects on brain function (Garcia-Segura, 2009). Several studies indicate that the older brains in comparison with younger brains respond in different ways to estrogen therapy. Less spines, including the alpha estrogen receptors, are on CA1 pyramidal cells in aged female rats compared to young females (Adams et al., 2002). Levels of sexual hormones are related to sexual cycle in females. Because of the short reproductive cycle of rodents named estrus cycle, they are used as ideal animal models for research on the reproductive cycle changes. Estrous cycle happens every