To elucidate the steroid-synthesis in the mammalian brain (i. e., neurosteroid), we immunohistochemically studied various kinds of steroidogenic cytochrome P450 enzymes in the rat brain. The primary antibodies used in this study were rabbit polyclonal antibodies to cholesterol side-chain cleavage (SCC), C21-hydroxylase (C21), 11β-hydroxylase (11β), 17α-hydroxylase/C17-20 lyase (17α) and aromatase (AROM). The immunoreactivities for the microsome enzymes, C21 and 17α were located in the neuronal cell-bodies and proximal parts of their fibers, while mitochondria enzymes, SCC and 11β were located in the cell-bodies and their fibers and terminals. These immunoreactivities were distributed in the limbic structures of prosencephalon including hippocampus and amygdaloid complex, the hypothalamus including preoptic area, the cerebellar cortex, and some of other regions. All the sets of these enzymes did not always coexist in the identical cells, however, in the hypothalamus and cerebellum these enzymes are thought to work one after another in adjacent cells, forming the “steroidogenic cellular circuits”. These findings strongly suggest that the steroidsynthesis occurs in the neurons; and the neurosteroids exist in the mammalian brain.