Immunoreactive calcitonin and calcium concentrations were determined on 581 plasma samples collected during 23 studies on 20 cows. Sample collections in each study was begun approximately 1 month prior to parturition and continued for about 1 month after parturition. The cows were grouped according to the degree of hypocalcemia encountered at parturition. The parturient paresis group consisted of 10 cows which developed severe hypocalcemia (3.91 plus or minus 0.22 mg/100 ml, mean plus or minus se) accompanied by paresis; the nonparetic hypocalcemic group consisted of 5 cows which developed severe hypocalcemia (5.70 plus or minus 0.03 mg/100 ml) but not paresis; and the control group consisted of 8 cows which experienced only a mild hypocalcemia (8.50 plus or minus 0.27 mg/100 ml) at parturition. In the prepartal period prior to the onset of hypocalcemia, the respective mean plasma calcium concentrations (plus or minus se) of the 3 groups were 10.1 plus or minus 0.11, 9.95 plus or minus 0.20, and 10.2 plus or minus 0.17 mg/100 ml. The development of severe hypocalcemia in the parturient paresis and nonparetic hypocalcemic groups was not accompanied by an increase in plasma calcitonin concentration. Furthermore, plasma calcitonin concentraion of these 2 groups was less than that of control cows during the parturient period as well as during the month before and the month after parturition. The plasma calcium nadir at parturition was positively related to the mean prepartal (encompassing the period from 30 days until 60 h before parturition) plasma calcitonin concentration (r = 0.57, t= 3.14, p less than 0.005); i.e., the lower the prepartal plasma calcitonin concentration the more severe the hypocalcemia at parturtion. These observations suggest that the development of hypocalcemia at parturition is not due to an increased secretion of calcitonin, but instead they suggest that parturient hypocalcemia may be associated with a diminished prepartal secretion of calcitonin.