The effect of disodium cromoglycate (cromolyn) in preventing the pulmonary dysfunction caused by the inhalation of barn and hay dust was studied in 5 horses with confirmed chronic obstructive pulmonary disease (COPD). The horses were studied before (Con) and after exposure to hay and dust allergens (Expos) and after pretreatment with cromolyn followed by exposure (Cr-Expos). There was a significant reduction in PaO2 from 86.8 +/- 8.3 to 73.1 +/- 8.8 when the horses were exposed to hay and dust allergens. The PaO2 after pretreatment with cromolyn and exposures was 78.1 +/- 5.5. There were no significant changes in PaCO2, FRC, pH and A-aDO2 when the Con, Expos, and Cr-Expos periods were compared. There were significant increases in VE from a control value of 77.9 +/- 18.2 to 128.7 +/- 55.1 and 133.7 +/- 17.1 L/min during the exposures, which was due primarily to increases in respiratory frequency. Respiratory dead space (VD/VT) increased from 0.55 +/- 0.10 to 0.71 +/- 0.07 and 0.65 +/- 0.05, and alveolar ventilation (VA) remained constant. Pulmonary resistance (RL) and transpulmonary pressure (Ptr) increased from a control of 0.77 +/- 0.28 cm H2O/L/sec and 7.73 +/- 3.38 cm H2O to 2.93 +/- 1.01 and 20.17 +/- 4.81 during the Expos period and tidal volume (VT) fell from 7.5 +/- 1.0 to 5.7 +/- 1.3 L. The pre-treatment with cromolyn before exposures significantly reduced the increase in RL and Ptr and returned VT to Con levels.