The change in circulating levels of immunoreactive parathyroid hormone (iPTH), measured with an N‐terminal specific radioimmunoassay, was examined during lactation in rats. In lactating rats consuming a diet containing 0.4% Ca (basic diet), serum iPTH was a) increased by an average of 53% between days 10 and 18 compared to the level of age‐matched nonlactating rats (24.7 ± 2.1 pg/ml vs 16.1 ± 0.8 pg/ml, mean ± SE, p < 0.01) and b) significantly higher in dams suckling large litters (10–15 pups) than in dams suckling small litters (3 pups) over the period 3–13 days of lactation. Lactating rats consuming a low calcium diet (0.04% Ca), had serum iPTH levels on days 16–18 of lactation approximately twice those of nonlactating rats fed the same diet and 73% higher than those of lactating rats fed the basic diet. Serum Ca concentrations were 22% and 10% lower in dams consuming the 0.04 and 0.4% Ca diets, respectively, than in the nonlactating controls fed the same diets. Regression analysis showed a significant (p < 0.001) negative correlation between iPTH and total serum calcium. Compared with nonmated controls, net mineral loss from femurs of dams consuming the 0.4% Ca diet was a) insignificant at day 6, b) 27% at day 15, and c) 34% at day 21 of lactation. Our data demonstrate that lactation in the rat is characterized by hyperparathyroidism that appears to be related to lactational intensity and that is accentuated when dietary calcium intake is restricted. The decrease in bone mineral during the period of elevated serum iPTH also suggests that parathyroid hormone plays a role in the bone loss normally seen in lactating rats.