This study was designed to test the hypothesis that women exhibit peaks of sexual activity at ovulation, as would be predicted from estrous effects in animals. Married women who used contraceptive devices other than oral contraceptives experienced a significant increase in their sexual behavior at the time of ovulation. This peak was statistically significant for all female-initiated behavior, including both autosexual and female-initiated heterosexual behavior, but was not present for male-initiated behavior except under certain conditions of contraceptive use. Previous failures to find an ovulatory peak may be due to use of measures of sexual behavior that are primarily determined by initiation of the male partner. Women using oral contraceptives did not show a rise in female-initiated sexual activity at the corresponding time in their menstrual cycles, probably owing to the suppression of ovulatory increases in hormone secretion by the oral contraceptives.