Low oxygen in the blood (hypoxemia) may occur in the neonate or women in the postpartum period. Administration of inhalation anesthetic may be required in this period. The purpose of this study was to evaluate the effect of 7 d of hypoxia on the neonatal rat pup and lactating dam without or with acute halothane anesthesia on serum calcium and calciotropic hormones. Ionized calcium was not altered by hypoxia or halothane administration. Hypoxia from birth had no effect on serum parathyroid hormone (PTH) in 7-d-old rat pups (48+/-4 pg/mL). Halothane increased PTH in rat pups (74+/-8 pg/mL). The effect of halothane was not augmented in hypoxic pups. Hypoxia for 7 d had no effect on serum PTH in lactating dams (23+/-3 pg/mL). Halothane resulted in an increase in PTH (106+/-17 pg/mL). When halothane was administered to hypoxic lactating dams, a striking increase in serum PTH was observed (401+/-50 pg/mL). We hypothesize that halothane and hypoxia alter parathyroid gland function by a direct effect on cellular calcium dynamics. This interaction may have clinical significance in hypoxic patients requiring general anesthesia.