A group of women were studied over two consecutive menstrual cycles, at about one week intervals. Twenty-one of these women had spontaneous ovulatory menstrual cycles, 6 women had anovulatory cycles, and 7 were regular users of oral contraceptives. All cycles were converted to classic 28 day cycles. On each visit, a record was obtained of the subject's galvanic skin responses (GSRs) to 21 successive auditory stimuli. It was found that, during ovulatory cycles, a marked change in the rate of habituation of the GSR occurred around day 14 and day 28. During days 1—14 the mean habituation point (number of GSR responses to sound before 3 successive nonresponses) was 12.4 SE 1.6. This dropped to 6.8 SE 1.6 during days 15-28 (2-tailed t test, p<0.001). This change was not found for women on oral contraceptives, who showed consistent habituation rates throughout the cycle (mean habituation point 7.2 SE 1.1). The evidence also suggested that women with anovulatory cycles differed from those with ovulatory cycles in that they did not show a chnage in their habituation point around midcycle.