In our previous study, we found a novel gene encoding a serine protease termed neuropsin (NP) which exhibited activity-dependent gene expression or repression in the mouse hippocampus (Chen et al., 1995). In the present study, we examined the ontogeny of NP mRNA expression by in situ hybridization in the brain. Weak hybridization signals were also observed in the choroid plexus at this stage in addition to neuronal labeling. The signals continued to show this localization pattern until postnatal day 12. After embryonic day 18, the number of hybridization signals localized in the neurons of the forebrain limbic area were more predominant than those in the lower brainstem. NP gene expression spread in the anterior olfactory nucleus, hippocampus, septal nuclei, diagonal band of Broca, amygdala and limbic cortex successively from early embryonic to adult stage, though signals in the other brain regions were gradually decreased after birth. Thus, the widespread localization and two types of expression pattern, constitutive or transient, suggest that NP is a multiple functional protein involved in development, neuronal plasticity and cerebrospinal fluid production.