A study of rod-like structures (RLS) was made in 173 pathological and in 67 normal brains. The pathological brains included cases with varied neuropathological conditions: vascular, metabolic, degenerative, infectious, traumatic, neoplastic, etc. The ages of the patients ranged from newborn to 97 years. RLS were found in 152 pathological brains (89%) and in 50 normal brains (75%). RLS were localized in all but one case, in Ammon's horn, specifically in Sommer's sector and in the stratum lacunosum beneath Sommer's sector. There was no correlation between any group of diseases studied and appearance or number of RLS. The number of RLS in Sommer's sector increased with advancing age. In the middle age, ihowever, the stratum lacunosum showed a higher number of RLS. The results of this study idicate that there is no significant relationship between RLS and any pathological condition and therefore that they represent non-specific changes, although a correlation with advancing age is probable. Although RLS appeared to be intracellular, their exact localization was not established.