Pulmonary reactivity to methacholine after equipotent β-blocking doses of propranolol and esmolol was compared in seven basenji-greyhound dogs during thiopental–fentanyl anesthesia. Equipotent doses of esmolol and propranolol were determined by both heart rate effects and by isoproterenol response curves. Propranolol (2 mg/kg) was administered intravenously as a bolus dose and esmolol (0.4–0.5 mg · kg-1· min-1) was administered by continuous infusion. Both esmolol and propranolol significantly decreased heart rate and mean arterial pressure (P< 0.001). Baseline pulmonary resistance and dynamic compliance did not change after the administration of either propranolol or esmolol. However, propranolol significantly shifted the methacholine dose-response curve to the left so that methacholine (0.3 mg/ml) increased pulmonary resistance by 7.9 ± 0.97 cmH2O · I-1· s-1(mean ± SEM of seven dogs) after pretreatment with propranolol but only 4.4 ± 0.89 cmH2O · I-1· s-1(P< 0.05) without propranolol pretreatment (control). Esmolol, however, did not significantly shift the methacholine dose-response curve. Methacholine (0.3 mg/ml) increased resistance 4.0 ± 0.89 cmH2O·I-1· s-1during esmolol administration. This study shows that at equipotent β1-blocking doses, propranolol, but not esmolol, produces significant increases in pulmonary reactivity to methacholine.