Using a balanced experimental design, 30 regularly cycling women not using oral contraceptives, aged 20 to 30 years, were assessed in a laboratory setting during the follicular, ovulatory, and luteal phases. Sexual arousal was measured by self-report and by photoplethysmographic recordings of vaginal vasocongestion. Plasma estradiol, progesterone, and testosterone were determined by radioimmunoassay. There were marked and consistent individual differences among the women in their sexual arousability. Subjective reports of sexual arousal did not differ among menstrual cycle phases, but physiologic arousal did vary with significantly higher mean levels of arousal occurring during the follicular and luteal phases than during the ovulation phase. While hormones fluctuated predictably with menstrual cycle phase, there were wide individual differences in absolute values among women. The correlations between estradiol, progesterone and testosterone and the subjective and objective assessments of arousability were mainly low and insignificant. Some evidence suggestive of a relation between testosterone and sexual arousability was observed when three subgroups of women were formed on the basis of relative average testosterone concentrations.