The area of cognitive research that has produced the most consistent sex differences is the area of spatial ability. Particularly, males usually perform better on mental rotation tasks than do females. One argument for these differences is that experience with spatial activity drives these differences, such that traditionally more masculine activities require more practice of spatial abilities. Another argument is biological in nature, such that there is either 1) a critical period of development that leads to differential lateralization of the brain, or 2) differential activation of the brain by circulating hormones. Performance on mental rotation tasks has been associated with right parietal activation levels, both during task performance and prior to performance during baseline recordings. The present study examined the relations among sex, age, EEG hemispheric activation (at the 10.5-13.5Hz. frequency band), and 2-dimensional mental rotation task ability. Nineteen eight-year-olds (10 boys) and 20 college students (10 men), had EEG recorded at baseline and while performing a mental rotation task. Men performed better on the mental rotation task than women, while there were no differences between boys and girls. After covarying for baseline EEG high alpha power values, EEG results during the mental rotation task indicated an interaction, with men exhibiting more activation (lower EEG power values at 10.5-13.5Hz) than women in the parietal and posterior temporal regions, while boys' and girls' power values 10.5-13.5Hz did not differ in the parietal or posterior temporal regions. Furthermore, during the baseline condition, men generally exhibited more activation (lower EEG power values at 10.5-13.5Hz) throughout all regions of the scalp. Results support the hypothesis that hormones, or hormonal influence, may result in a biological change which affects both brain activation and performance on mental rotation tasks. / Master of Science