Sex differences in human cognitive and motor skills may in part be due to organizational or activational effects of sex hormones on the brain. In this study, an extensive battery of cognitive and motor tests was administered to normally cycling women at two phases of the menstrual cycle, in order to detect any hormone-mediated changes in performance. Results confirmed changes across the menstrual cycle on a variety of speeded manual and articulatory measures, and on some nonverbal/spatial tests. The results provide qualified support for the hypothesis that the high levels of gonadal steroids present at the luteal phase of the cycle may facilitate skills favoring females, but be detrimental to skills favoring males. The implications of these results for research in the area of human sex differences are discussed.