Calcitriol is used to treat hyperparathyroidism in hemodialysis patients. Calcitriol treatment, either through a reduction in parathyroid hormone (PTH) levels or direct effect on bone, decreases the osteoblast and osteoclast surface and bone formation rate. Our study of 13 hemodialysis patients was designed to evaluate whether calcitriol treatment changed the rate of spontaneous recovery from hypocalcemia induced by a low-calcium dialysis. Calcitriol treatment decreased basal PTH levels from 614 +/- 84 to 327 +/- 102 pg/mL (P < 0.001) and maximal PTH levels from 1,282 +/- 157 to 789 +/- 161 pg/mL (P < 0.001), but the rate of serum ionized calcium recovery from hypocalcemia did not change. When the 13 patients were separated based on the median age of 64 years, the predialysis serum ionized calcium level was less in the younger (group I, 44 +/- 6 years; n = 6) than older (group II, 68 +/- 1 years; n = 7) patients (1.05 +/- 0.03 v 1.22 +/- 0.03 mmol/L, respectively; P < 0.01) despite similar basal (group I, 595 +/- 122 pg/mL v group II, 629 +/- 96 pg/mL) and maximal (group I, 1,114 +/- 299 pg/mL v group II, 1,425 +/- 141 pg/mL) PTH levels. Before calcitriol treatment, the rate of serum ionized calcium recovery from induced hypocalcemia was greater (P < 0.05) for similar PTH levels in the older than younger patients. After calcitriol treatment, despite a similar reduction in PTH levels, the rate of calcium recovery increased (P < 0.05) in the younger patients but did not change in the older patients. We also observed that toward the end of the low-calcium hemodialysis, PTH values decreased even though serum ionized calcium level continued to decline when the rate of calcium reduction slowed. In addition, hysteresis, defined as a lower PTH value during the recovery from hypocalcemia than during the induction of hypocalcemia for the same serum calcium concentration, was present during the spontaneous recovery from hypocalcemia. In conclusion, in the hemodialysis patient: (1) age appeared to affect the bone response to PTH and calcitriol treatment, (2) the PTH response to hypocalcemia was affected by a deceleration in the rate of calcium decrease, and (3) hysteresis of the PTH response to hypocalcemia occurred during the spontaneous recovery from hypocalcemia.