1. 1. The mid-menstrual cycle breast temperature ( T Br) peak in women earlier described by Nassar & Smith (1975) has been more closely evaluated and compared with menstrual cycle variations in other peripheral regions. 2. 2. Alignment of the T Br, heat flow and conductance data by the day of the T Br peak showed a significant rise and fall in these three variables, the peak occurring 2–4 days prior to the estimated day of ovulation and the nadir usually within 1 day of estimated ovulation. The relative changes in these data during the mid-cycle peak proved significant and distinguished it from other cyclic variations or elevations later in the cycle. This phenomenon was not found to occur at peripheral areas of the arm, thigh, calves and forehead. 3. 3. Rectal temperature ( T Re) remained stable at the time of the T Br peak, with a slight elevation following estimated ovulation. 4. 4. Progesterone assays clearly indicated the occurrence of ovulation in 70% of the subjects studied. 5. 5. It is noted that the T Br pattern very closely resembles that of plasma oestradiol levels established for humans during the menstrual cycle, and it is hypothesized that the breast thermal response is due to cyclic changes in oestrogen levels leading to an increase in blood flow to that area.