Adult neural stem cells (NSC) proliferate and differentiate depending on the composition of the cellular and molecular niche in which they are immersed. Until recently, microglial cells have been ignored as part of the neurogenic niche. We studied the dynamics of NSC proliferation and differentiation in the dentate gyrus of the hippocampus (DG) and characterized the changes of the neurogenic niche in adrenalectomized animals (ADX). At the cellular level, we found increased NSC proliferation and neurogenesis in the ADX animals. In addition, a morphologically distinct subpopulation of NSC (Nestin+/GFAP-) with increased proliferating profile was detected. Interestingly, the number of microglial cells at stages 2 and 3 of activation correlated with increased neurogenesis (r2 = 0.999) and the number of Nestin-positive cells (r2 = 0.96). At the molecular level, transforming growth factor beta (TGF-beta) mRNA levels were increased 10-fold in ADX animals. Interestingly, TGF-beta levels correlated with the amount of neurogenesis detected (r2 = 0.99) and the number of stage 2 and 3 microglial cells (r2 = 0.94). Furthermore, blockade of TGF-beta biological activity by administration of an anti-TGF-beta type II receptor antibody diminished the percentage of 5-bromo-2'-deoxyuridine (BrdU)/PSA-NCAM-positive cells in vivo. Moreover, TGF-beta was able to promote neurogenesis in NSC primary cultures. This work supports the idea that activated microglial cells are not pro- or anti-neurogenic per se, but the balance between pro- and anti-inflammatory secreted molecules influences the final effect of this activation. Importantly, we identified an anti-inflammatory cytokine, TGF-beta, with neurogenic potential in the adult brain.