This study was designed to (1) examine the ability of fetal diencephalic neurons cultured in vitro to express aromatase mRNA and (2) evaluate the involvement of several environmental factors which may regulate the development and differentiation of the neurons in the central nervous system. Brain cells from fetal mice at various developmental stages were cultured as tissue slices and as primary monolayer cells, and the expression levels of aromatase mRNA in the cultured cells were measured by a quantitative reverse transcription-polymerase chain reaction method using an internal standard. On cultured slices of the diencephalic region from fetal mice on embryonic day 12 (E12), E13, or E15 on collagen-coated membranes, the expression level of the mRNA continued to increase for the initial 2-3 days as that in vivo. Time-dependent increase of aromatase mRNA was also observed for 3 days in E13 neuronal cells dissociated with papain and cultured on poly L-Lys-coated dishes in serum-free medium. However, no significant time-dependent increase of the aromatase mRNA level was observed in E10 or El 1 brain cells cultured by either method. These findings suggest that the developmental increase of aromatase mRNA in diencephalic neurons is an endogenous characteristic and probably genetically determined after E12.