The effects of the menstrual cycle on cognitive functions were investigated using simple verbal and spatial tasks. Eight healthy young women with a regular, established 28-day menstrual cycle and the occurrence of ovulation on day 14 were tested four times during one cycle. Ten women on non-tricyclic birth control pills were also tested weekly during one cycle. Both groups were matched in age, handedness and education. No significant difference in spatial ability was found but improved verbal working memory, as measured by the verbal span score, was associated with periods of high estrogen levels.