&NA;The influence of inhalational anesthetics on histamine released during bronchospasm induced by Ascaris antigen aerosol was studied in Basenji‐Greyhound (BG) dogs. Five BG dogs were anesthetized with thiopental and halothane on separate occasions and challenged with Ascaris antigen aerosol for five minutes. Pulmonary resistance (RL), dynamic compliance (Cdyn). and arterial plasma histamine levels were measured over 30 min following antigen exposure.Prior to antigen challenge, RL, Cdyn, and histamine levels were similar in thiopental‐ and halothane‐anesthetized dogs. The peak change in RL, and Cdynoccurred 10 min after the start of antigen challenge. RL, increased 3.0 ± 0.4‐fold (mean ± SEM) in thiopental‐anesthetized dogs as compared with 1.6 ± 0.2‐fold in halothaneanesthetized dogs (P < 0.05). Cdyndecreased to a similar extent in both groups, with maximal decreases of 0.53 ± 0.08 and 0.53 ± 0.09 times the prechallenge value for thiopental‐ and halothane‐anesthetized dogs, respectively. Plasma histamine levels peaked at 5 min. Plasma histamine increased to 8.0 ± 5.0 ng/ml in thiopental‐anesthetized dogs compared with 10.7 ± 4.7 ng/ml in halothane‐anesthetized dogs. Histamine levels were not significantly different during or after antigen challenge between thiopental‐ and halothane‐anesthetized dogs.Halothane, but not thiopental, significantly attenuated the increase in RLprovoked by antigen challenge. However, arterial plasma histamine levels were similar during both anesthetics. Therefore, the authors conclude that the protective effect of halothane on airways was not due to an inhibition of release of histamine from mast cells.