To further our understanding of cognitive sex differences, we studied the relationship between menstrual phase (via serum estradiol and progesterone levels) and cognitive abilities and cognitive performance in a sample of medical students in eastern Turkey. As expected, we found no sex differences on the Cattell "Culture Fair Intelligence Test'' (a figural reasoning test), with females scoring significantly higher on a Turkish version of the Finding A's Test (rapid word knowledge) and males scoring significantly higher on a paper-and-pencil mental rotation test. The women showed a slight enhancement on the Finding A's Test and a slight decrement in Cattell scores during the preovulatory phase of their cycle that (probably) coincided with a rise in estrogen. There were also small cycle-related enhancements in performance for these women on the mental rotation test that may reflect cyclical increases in estrogen and progesterone. Additional analyses showed an inverted U-shaped function in level of estradiol and the Cattell Test. Finally, for women who were tested on Day 10 of their menstrual cycle, there was a negative linear relationship between their Cattell scores and level of progesterone. Stereotypes about the cognitive abilities of males and females did not correspond to performance on the mental rotation or Finding A's Test. so the sex-typical results could not be attributed to either stereotype threat or stereotype activation. For practical purposes, hormone-related effects were generally small. Variations over the menstrual cycle do not provide evidence for a "smarter'' sex, but they do further our understanding of steroidal action on human cognitive performance. (C) 2001 Academic Press.