Male (n=18) and female (n=16) subjects completed a three-dimensional mental rotation task (MRT). The hormonal and MRT data were collected from female subjects at low (menstruation) and high (ovulation) estrogen phases. Each subject's reaction time (RT) and error rate (ER) were plotted to give the slope and intercept of the functions, relating performance to angular disparity. Males showed faster RT and lower ER compared to females at each menstrual phase; the difference arises from the slope. These findings suggest that there is a sex difference in mental rotation at each menstrual phase.