In order to describe circadian temperature rhythms in relation to female ovulatory cycles, a study was conducted in which continuous rectal temperature was monitored in healthy women at two phases of their menstrual cycles. Results indicate that, in addition to an increase in the mesor, there is a significant dampening of the temperature amplitude during the luteal (postovulatory) phase compared to the follicular (preovulatory) phase. In comparison with studies of male subjects, the average acrophase for females may occur about 2 hr earlier.Results from this study provide descriptive normative data, controlling for menstrual cycle phase, to which female clinical populations can be compared. The use of circadian temperature rhythm as a possible noninvasive diagnostic indicator of ovulation is also discussed.As women ovulate and then menstruate during a monthly cycle, gonadal hormone levels rise and fall. One of these gonadal hormones, progesterone, is secreted from the corpus luteum during ovulation and throughout the luteal phase of the menstrual cycle. The level of progesterone drops dramatically with the onset of menses and remains at negligible levels until the next ovulation occurs. Progesterone's thermogenic effects (Little et al., 1974) allow women to use basal body temperature (BBT) as a simple method for determining ovulatory cycles (Moghissi, 1980). During the luteal phase of the menstrual cycle, from ovulation until the next menses, an ovulating female will exhibit higher early morning oral temperatures when compared to the preovulatory, or follicular, phase. Yet some BBT charts are difficult to interpret, even by highly qualified specialists (Quagliarello & Arny, 1986). This difficulty may be accounted for by individual differences in the circadian temperature peaks and nadirs related to sleep and wakefulness.The purpose of this study was to describe circadian temperature rhythms in healthy women during the low-and high-progesterone phases of the menstrual cycle. Although many circadian rhythm researchers have used female subjects in their studies, few have controlled for the phase of the menstrual cycle in their study designs or in the interpretation of their findings.