The serum concentrations of total calcium and 1,25-dihydroxyvitamin D3 were measured in seven women during a menstrual cycle. A rise in the serum level of 1,25-dihydroxyvitamin D3 was observed on day 15 of the cycle without a detectable change in the serum calcium level. The level of 1,25-dihydroxyvitamin D3 on day 15 was double the value at days 1 and 8 of the cycle. The peak of 1,25-dihydroxyvitamin D3 on day 15 was not observed in five women ingesting oral contraceptives, suggesting that ovulation or its hormonal control was involved in the peak on day 15. A small but significant fall in the serum calcium concentration occurred in the women ingesting oral contraceptives. In view of these findings the serum concentrations of 1,25-dihydroxyvitamin D in women with functioning ovaries must be interpreted in the context of the stage of the menstrual cycle when the blood sample was obtained.