Parathyroid function was assessed by calcium infusions (4-8 h) in 16 patients with chronic renal insufficiency being treated by long-term hemodialysis. The concentrations of two immunoreactive species of parathyroid hormone in plasma (iPTH-9, mol wt 9500; iPTH-7, mol wt 7000) were estimated by radioimmunoassays utilizing two relatively specific antisera. Control values of the smaller species, iPTH-7, were uniformly high, whereas values of iPTH-9 were normal in 12 of 19 studies. Response of iPTH-7 to calcium infusions was variable, with significant decreases occurring only five times in 27 infusions. Concentrations of iPTH-9, however, decreased during every calcium infusion. In contrast to these acute responses, five of six patients studied during periods of dialysis against both low (< 6 mg/100 ml) and high (7-8 mg/100 ml) calcium concentrations in the dialyzate showed a decrease in values of iPTH-7 during the period of dialysis against the higher calcium concentration. It is concluded that plasma concentrations of iPTH-9 reflect primarily the moment-to-moment secretory status of the parathyroid glands, while concentrations of iPTH-7 reflect more closely chronic parathyroid functional status. It is further concluded that the failure of iPTH-7 to decrease during induced hypercalcemia should not be equated with autonomy of parathyroid gland function.