Although the benefits of regular exercise have been known for several organs and systems, the effect on brain health is still under debate during aging. It has been widely accepted that type and duration of exercise play a critical role in the maintenance of cognitive function and memory. Ultramarathon (UM) has become a popular sport in recent years because of its benefits. This study aimed to determine the acute effect of 25 km half-marathon (HM) and 64 km UM on the hippocampus, especially on its subregions that are responsible for cognitive function and memory. A total of 15 healthy male runners aged between 40-60 years were grouped as HM (n=7) and UM (n=8). Brain scans were obtained with 3T magnetic resonance imaging (MRI) before and after the race. Volumetric differences in the subregions of the hippocampus were compared by using a paired-sample t-test in HM and UM groups before and after the race. As a result, the volume of subiculum subregion was increased in both HM and UM groups, whereas volumetric increase in total hippocampus and CA1-3 subregions were observed only in the UM group. The volume of DG remained unchanged in both HM and UM groups after the race. It can be concluded that endurance running causes an increase in hippocampal volume in healthy male runners. However, a certain threshold is needed to stimulate the CA1-3 subregion of the hippocampus. In conclusion, endurance running may prevent age-dependent atrophy in the hippocampus which has a key role in cognitive function and memory.