Recent data suggest that males and females perform spatial tasks in different ways. Men's performance may be better because they create a more accurate representation of the Euclidean and geometric properties of the environment than women, whose representations may be more dependent on available landmarks. Moreover, in women, spatial navigation performance may be influenced, among other possibilities, by hormonal fluctuations. The present study sought to determine the effects of two phases of women's hormonal cycle on spatial abilities in a navigation task, and more generally to examine possible differences between men and women at these two phases of the feminine cycle. Participants navigated in a three-dimensional virtual environment in order to learn an irregular arrangement of target-cylinders of different colours, and of landmark objects. In subsequent tests the cylinders were made of the same colour and the landmarks were either present or absent. Participants had to locate target cylinders, identified by their previously-learned colour. During the menstrual phase (low level of estrogens and progesterone), women were as good as men in both types of tests (with or without landmarks). By contrast, during the post-ovulatory phase, their spatial performance was specifically impaired on the tests without landmarks (that were the most difficult for both sexes). This study demonstrates that when only geometric information is available to solve the task, women's spatial performance is poorer at the end of the hormonal cycle than at its beginning.