A study designed to investigate how the cellular laterality in rostro-caudal direction changes due to age and sex was presented. It is a well-known fact that asymmetry is an important issue in understanding brain functions. Frontal sections of 5 Pin thickness with 200 mum intervals were taken from both dorsal hippocampi of 4- and 20-week-old male and female rats. Sections were stained with hematoxylin-eosin and the levels of sections were determined according to a stereotaxic atlas. In each level, numerical density of pyramidal cells of the right and left area CA1, CA3 and CA4 of dorsal hippocampi were estimated by means of counting pyramidal cells under 10 x 40 magnification. Numerical cell density obtained from the right and left dorsal hippocampi of male and female rats were compared. Results are as follows: 1) Numerical cell density in the left and right dorsal hippocampi of 20-week female rats was not high than 4-week female dorsal hippocampi (p>0.05). 2) Difference between 4-week-old female and male dorsal hippocampi of right and left CA1, CA3 and CA4 areas were not raised a significant level (p> 0.05). Differences between 20-week-old group of male and female rats with the exception of female CA4 area were not also significant. 3) Right and left dorsal hippocampi CA4 areas of the 20-week-old group showed a higher numerical density than the same area of the 4-week-old group (p<0.007). This difference may show a postnatal neurogenesis in area CA4. The results, which are summarized above, may be helpful in understanding the mechanisms of human brains.