The effects of anesthetics on serum parathyroid hormone (PTH) concentrations were determined by a new homologous two‐site immunoradiometric assay for rat PTH. Serum PTH concentrations (mean ± SE) from ether‐anesthetized rats (14.7 ± 1.5 pg/ml, n = 22) were not significantly different from those of decapitated unanesthetized female rats (13.0 ± 1.8 pg/ml, n = 21). Serum PTH concentrations in pg/ml (n = 4–14) for other anesthetics tested were: ketamine, 12.5 ± 1.1; Na pentobarbital, 23.3 ± 2.4; methoxyflurane (inhalation), 42.2 ± 6.8; and xylazine combined with ketamine, 51.4 ± 11.3 pg/ml. The latter two concentrations were significantly (p < 0.001) higher than the values for all other anesthetics and decapitation. Elevation of serum PTH induced by pentobarbital or ketamine + xylazine increased with time under anesthesia. Neither serum Ca²⁺ concentrations nor pH differed among any of the groups. We conclude that anesthesia induced by pentobarbital, methoxyflurane, or ketamine + xylazine in rats leads to a marked elevation of serum PTH levels that appears to be related to the duration of anesthesia and not due to any measurable fall in serum Ca²⁺.