The suckling period, a time when rapid growth of the skeleton requires retention of Ca, was selected for investigation of a possible role for calcitonin (CT) in normal physiology. Serum concentrations of Ca, P and immunoreactive CT were examined in 14 day old rat pups during fasting and suckling. Litters of 6-10 pups each either were allowed to nurse ad lib or were separated from their mothers (fasted) for 4-12 h. After fasting, half of each litter was allowed to suckle for 30 min-2 h. Babies nursing ad lib and with milk curds in the stomach showed a serum Ca of .apprx. 10.5 mg/dl and a high serum CT of several thousand pg/ml. These values were not changed by 4 h of fasting, although the stomach had essentially emptied of milk curds. In babies fasted for 8 or 12 h, serum Ca was .ltoreq. 10.5 mg/dl and serum CT fell to low or undetectable levels (< 120 pg/ml). Littermates which had been fasted 8-12 h and then allowed to suckle for 30-60 min ingested 1-2 ml milk and showed a rise in both serum Ca (> 11 mg/dl) and serum CT as well as increased release of immunoreactive gastrin. By 2 h after the onset of feeding serum CT was still high and serum Ca had returned to normal in thyroid-intact babies, but the reversal of serum Ca was slowed and serum CT was undetectable in acutely thyroidectomized pups. Lactating mother rats used in the study also were found to have high circulating levels of CT. Serum CT is high in the baby rat during suckling. Serum CT falls to low levels after 8-12 h of fasting. After fasting serum CT rises in response to suckling and a rise in serum Ca. CT, secreted during suckling, may help to minimize postprandial hypercalcemia and, by acting to restict efflux of Ca from the skeleton, help to conserve dietary Ca so that it may be utilized efficiently without causing large perturbations in blood Ca.