Calcium metabolism during the menstrual cycle was studied in seven women from whom fasting blood samples were drawn daily or every other day throughout ovulatory cycles. Total calcium (Ca), ionic calcium (Ca++), magnesium (Mg), phosphorus (P), and immunoreactive parathyroid hormone (PTH) and calcitonin (CT) were measured. LH levels were used to date each cycle and progesterone levels were used to confirm ovulation. Plasma estradiol was measured in two of the subjects. In six subjects with cycle lengths of 27-31 days, PTH levels rose progressively through the follicular phase to a peak at or slightly before the LH surge, then fell progressively through the luteal phase; peak PTH levels were 30-35% above early follicular and late luteal values. CT levels were also highest at midcycle, but the CT pattern was somewhat more variable than that of PTH. Ca++ tended to fall until 3-4 days before ovulation and then to increase, while Ca, Mg, and P exhibited no particular pattern. One subject experienced a prolonged (44 day) ovulatory cycle characterized by three distinct PTH peaks, each of which coincided with elevations in plasma estradiol level. These results represent the first report of menstrual cyclicity in calcium-regulating hormones. The timing suggest an estrogen effect and it is hypothesized that estrogen inhibits PTH-induced bone resorption, lowering serum Ca++, which in turn provokes a compensatory PTH output. With the decline of the preovulatory estrogen peak, Ca++ levels rise and PTH secretion falls. Alternatively, it is possible that the primary action may be an estrogen-induced rise in CT release, causing hypocalcemia and consequent PTH output. Cyclic changes in PRL release or vitamin D metabolism might also be involved.