The present study investigated the expression pattern of steroidogenic acute regulatory protein (StAR) mRNA in several brain regions and peripheral endocrine organs using Northern blot, RT-PCR and in situ hybridization. StAR mRNA in the adrenal gland was detected at birth and decreased for 2 weeks postnatally. In gonads, it was also detected at birth though at a lower level than adrenal, and was maintained until week 3. Thereafter StAR mRNA expression in both endocrine organs was increased. Though the amount of StAR mRNA in the brain was much less than that of peripheral endocrine organs, it was expressed from birth and, in general, appeared to gradually increase during postnatal development. A gradual increase was found in the hypothalamus, while a sigmoidal expression was shown in the olfactory bulb. The increased expression of StAR mRNA in the postnatal period suggests that it might have a role in the regulation of neurosteroidogenesis needed in neuronal cell growth and differentiation in postnatal rat brain development.