We measured serum concentrations of calcium-regulating hormones and osteocalcin in 20 women longitudinally throughout pregnancy, 1,25-Dihydroxyvitamin D levels were high early in pregnancy and increased with advancing gestation. Parathyroid hormone and osteocalcin levels were low in early pregnancy. They declined toward the middle of pregnancy, but increased in late pregnancy. The serum osteocalcin level correlated with the parathyroid hormone level. The synthesis of osteocalcin by osteoblasts is stimulated by the action of 1,25-dihydroxyvitamin D, and serum osteocalcin levels are also related to the levels of parathyroid hormone. During early and mid pregnancy, the stimulatory effect of 1,25-dihydroxyvitamin D on the synthesis of osteocalcin may be overridden by the inhibitory effect of declining parathyroid hormone levels. The increase in osteocalcin level in late pregnancy may be a consequence of increasing levels of both parathyroid hormone and 1,25-dihydroxyvitamin D.