Steroids can be synthesized de novo in the nervous system that influence the brain development and many neurophysiological functions. Neurosteroids have the ability to change neuronal excitability by modulating neurotransmitter receptors such as γ-aminobutyric acidA (GABAA) and N-methyl-D-aspartate (NMDA) receptors. Some neurosteroids, including pregnenolone (PREG), dehydroepiandrosterone (DHEA), progesterone (PROG) and allopregnanolone (Allo), have been shown to prevent neuronal cell death elicited by different stimuli. Besides, these steroids also affect neurite growth during development. In this study, we utilize knockout mice disrupted with Cyp11a1, the key gene controlling steroidogenesis, to elucidate the role of endogenous steroids in the neuronal survival and neurite outgrowth of brain. Cyp11a1 null mice lack the synthesis of steroids and die about postnatal 6 days. We used TUNEL assay to investigate the apoptosis in various brain areas of wild-type (WT) and knockout (KO) mice on postnatal day 5. Our data showed that when compared with the wild-type littermate, the number of TUNEL-positive cells in cerebral cortex, hippocampus, corpus callosum, cingulum bundle and hypothalamus were higher in the Cyp11a1 KO mice. It indicated that steroid deficiency may enhance the apoptosis in the developing brain. To understand the molecular pathway, we analyzed the expression of procaspase-3、Bcl-2 and Bax. Besides we examined the activation of caspase-3 by colorimetric assay in hippocampus and hypothalamus. However, there is no significant difference in the levels of these proteins and the activity of caspase-3 between WT and KO mice. On the other hand, we used Golgi-Cox stain to study the dendritic morphology of hippocampal pyramidal cells. Total dendritic length, the number of nodes and segments between WT and KO mice were not significantly different; however, the pattern of dendritic branching and the dendritic arbor distribution in KO mice were simpler than WT mice. These studies suggested that steroids may play a role in regulating cell survival and the hippocampal dendritic outgrowth in developing brain.