To further define control of mineral homeostasis during lactation, 28 lactating (L) and 20 nonlactating (NL) women were studied at 6 weeks post partum. Serum and urine calcium, magnesium, and phosphorus were normal and the same in both groups. 25-Hydroxyvitamin D (25-OHD) was the same (L, 18.6 +/- 4.8; NL, 17.0 +/- 5 ng/ml) in spite of a twofold higher intake of vitamin D in the lactating group. The serum fractions containing 24,25-dihydroxyvitamin D (24,25(OH)2D) were lower than in nonpregnant adults in both groups but significantly lower (P less than 0.05) in lactating than in nonlactating women (L, 1.67 +/- 1.7; NL, 2.46 +/- 1.3 ng/ml). 1,25-Hydroxyvitamin D (1,25-(OH)2D) was normal in both groups (L, 25.8 +/- 8.6, NL, 31.8 +/- 8.1 pg/ml). Parathyroid hormone (PTH) was normal in both groups (L, 5.6 +/- 2.0; NL, 7.2 +/- 1.9 microliters Eq/ml), and calcitonin (HCT) was equally detectable. As expected, prolactin was higher in the lactating group (L, 46 +/- 36; NL, 14.3 +/- 14.9 ng/ml). Serum prolactin levels had no correlation with serum 1,25(OH)2D. Estradiol was significantly (P less than 0.02) lower in lactating women (L, 78 +/- 23; NL, 105 +/- 28 pg/ml). One could speculate that the lower levels of estradiol in the presence of low 24,25(OH)2D and normal HCT allow bone resorption to occur at a rate adequate to supply the mineral needs of lactation without elevations of either PTH or 1,25(OH)2D.