A recent report demonstrated that in human aging brain after menopause/andropause luteinizing hormone (LH) is localized in the cytoplasm of pyramidal neurons of hippocampus and a significant increase of LH is also detected in the cytoplasm of pyramidal neurons and neurofibrillary tangles of Alzheimer's disease brain compared to age-matched control brain. It was suggested that the decreased steroid hormone production and the resulting LH expression in the neurons vulnerable to Alzheimer's disease pathology may have some relevance to the development of Alzheimer's disease. It is, however, unclear whether the presence of LH in neurons of human aging and Alzheimer's disease brain is due to intracellular LH expression or to LH uptake from extracellular sources, since gonadotropins are known to cross the blood brain barrier. Moreover, there is no report by using the brain of experimental animal that LH is expressed in such neurons as found in the human brain. In the present study, we found that LH immunoreactivity is localized in the pyramidal neurons of cerebral cortex and hippocampus of 12 and 18 months old rats but can not detect any immunoreactivity for LH in the young adult (3-5 months old) rats. To confirm that these LH immunoreactivity results from de novo synthesis in the brain but not the uptake from extracellular space, we performed RT-PCR and found that mRNA for LH is detected in several regions of brain including cerebral cortex and hippocampus. These findings suggest us that LH expression in old rat brain may play an important role in aging process of rat brain.