Parathyroid hormone (PTH) concentrations were measured by radioimmunoassay in sera from 75 women during all stages of pregnancy. In addition, PTH was determined in 14 women with well established lactation, and in 16 nonpregnant women of childbearing age. Serum PTH in the nonpregnant controls was 35 ± 4 (mean ± 1 se) with a range of 20–60. There was a significant decrease of serum PTH during the 20–24th weeks. Thereafter, a significant progressive rise occurred to a maximum value of 76 ± 5 (mean ± 1 se) during the 36–40th weeks. Seventeen of 20 women in their 9th month of pregnancy had values clearly above the normal range. In ten nonlactating women, 4–8 weeks postpartum, PTH was normal. In 14 women lactating 3 weeks to 27 months, PTH concentration was also normal. A slight decrease in total serum calcium concentration of 0.2 mg/100 ml was noted to occur between the 16th and 36th weeks, but this was not correlated with the rise in PTH. The data indicate that significant maternal hyperparathyroidism regularly occurs during the last trimester of normal pregnancy. Although the specific signal for parathyroid hypersecretion was not identified, the rise in PTH was best correlated with the demands of the growing fetal skeleton upon maternal calcium stores.