Parathyroid hormone (PTH) secretion rate was measured in 16 anesthetized calves by using a technique involving radioimmunoassay of parathyroid venous blood which was collected during timed intervals and measured volumetrically. The calves ranged in age from 2-14 weeks. Plasma calcium concentration was altered by infusion of solutions of CaCl2 or disodium ethylenediamine tetracetate (Na2 EDTA) into the jugular vein. When plasma calcium concentrations exceeded 10.5 mg/100 ml, a basal, non-suppressible secretion rate of 0.3 ng/kg/min was maintained despite the induction of hypercalcemia. Slight changes in secretion rate were observed in response to changes of plasma calcium in the range between 9 and 10.5 mg/100 ml. Below 9 mg/100 ml, a small decrease in plasma calcium concentration evoked a pronounced increase in secretion rate. A maximal secretion rate of about 5.5 ng/kg/min was attained at a plasma calcium concentration of approximately 7.5 mg/100 ml and it was not increased by more severe hypocalcemia. These observations confirm the sigmoidal relationship between PTH secretion rate and plasma calcium concentration which was previously suggested by measurement of PTH concentration in peripheral plasma of hypocalcemic, parturient cows.