We determined the nuclear volume of fibroblasts of the normal mammary lobular stroma during the follicular and luteal phases of the menstrual cycle. Twenty patients aged 15 to 35 years and eumenorrheic for at least 6 months were randomly assigned to 2 groups, i.e., 10 women in the follicular phase and 10 in the luteal phase. The nuclear volume was 34.4 micron 3 and 98.8 micron 3 for the follicular and luteal phases, respectively, with the difference being statistically significant (p < 0.05). These data suggest a higher metabolic activity in the mammary intralobular stroma during the luteal phase of the menstrual cycle, probably due to a synergistic action of estradiol and progesterone.