The evaluation of airway resistance (R_aw) in conscious mice requires both end-expiratory (V_e) and tidal volumes (V_t) (Ref. 22). In anesthetized BALB/c mice we measured lung area (A_L) from ventral-to-dorsal x-ray images taken at FRC (V_e) and after air inflation with 0.25 and 0.50 ml (V_L). Total lung volume (V_L) described by equation: V_L = V_L + V_FRC = KA_L^1.5 assumed uniform (isotropic) inflation. Total V_FRC averaged 0.55 ml, consisting of 0.10 ml tissue, 0.21 ml blood and 0.24 ml air. K averaged 1.84. In conscious mice in a sealed box, we measured the peak-to-peak box pressure excursions (P_b) and x-rays during several cycles. K was used to convert measured A_L^1.5 to V_L values. We calculated V_e and V_t from the plot of V_L vs. cos ( - ). Phase angle was the minimum point of the P_b cycle to x-ray exposure. , phase difference between the P_b and V_L cycles, was measured from P_b values using room and body temperature humidified box air. A similar analysis was used after aerosol exposures to bronchoconstrictor methacholine (Mch), except that depended also on increased R_aw. In conscious mice, V_e doubled after Mch (50-125 mg/ml) exposure with constant V_t, frequency (f), P_b and R_aw. In anesthetized mice, in addition to an increased V_e, repeated 100 mg/ml Mch exposures increased P_b and R_aw and decreased f to apnea in 10 minutes. Thus conscious mice adapted to Mch by limiting R_aw, while anesthesia resulted in airway closure followed by diaphragm fatigue and failure.