New research published in the Proceedings of the National Academy of Sciences shows that adults who exercise at least 40 minutes a week have a larger volume of neurons in the hippocampus. The hippocampus is a structure which is heavily involved in memory. Read more about this study over at NPR.
Another study published by PNAS shows that older adults who exercised at moderate intensity over a year gained volume in their hippocampuses. Read more about this study over at Science Daily.