Functional residual capacity (FRC), tidal volume (VT), and frequency (f) were compared in 23 rats while either awake and unrestrained or anesthetized. FRC was determined from gas compression with closed airway inside a cone-shaped body plethysmograph. In the awake state (mean +/- SD), FRC was 1.02 +/- 0.22 ml/100 g, VT was 0.38 +/- 0.06 ml/100 g, and f was 142 +/- 22 breaths/min. During anesthesia, FRC decreased (P less than 0.01) to 52.9% of awake values, VT increased (P less than 0.01) to 147.4%, and f decreased (P less than 0.01) to 71.8%, leaving minute ventilation almost unchanged. An additional seven rats were used to examine postural effects on FRC during anesthesia, and in another seven animals pleural pressure changes were monitored. Dynamic lung compliance (0.80 ml . kg-1 X cmH2O-1) was not altered by anesthesia, but the pressure-volume curve was shifted 6 cmH2O higher. Thoracic compression, followed by a time-dependent effect of volume history, may account for the major change in FRC. The remainder of the decrease in FRC may be due to lower breathing frequency, loss of inspiratory muscle activity, and/or less airway resistance after anesthesia. Peak diaphragmatic electromyogram per unit VT was shown to increase almost linearly with FRC, indicating that diaphragmatic efficiency was decreased as lung volume was elevated. Functional residual capacity (FRC), tidal volume (VT), and frequency (f) were compared in 23 rats while either awake and unrestrained or anesthetized. FRC was determined from gas compression with closed airway inside a cone-shaped body plethysmograph. In the awake state (mean +/- SD), FRC was 1.02 +/- 0.22 ml/100 g, VT was 0.38 +/- 0.06 ml/100 g, and f was 142 +/- 22 breaths/min. During anesthesia, FRC decreased (P less than 0.01) to 52.9% of awake values, VT increased (P less than 0.01) to 147.4%, and f decreased (P less than 0.01) to 71.8%, leaving minute ventilation almost unchanged. An additional seven rats were used to examine postural effects on FRC during anesthesia, and in another seven animals pleural pressure changes were monitored. Dynamic lung compliance (0.80 ml . kg-1 X cmH2O-1) was not altered by anesthesia, but the pressure-volume curve was shifted 6 cmH2O higher. Thoracic compression, followed by a time-dependent effect of volume history, may account for the major change in FRC. The remainder of the decrease in FRC may be due to lower breathing frequency, loss of inspiratory muscle activity, and/or less airway resistance after anesthesia. Peak diaphragmatic electromyogram per unit VT was shown to increase almost linearly with FRC, indicating that diaphragmatic efficiency was decreased as lung volume was elevated. Functional residual capacity (FRC), tidal volume (VT), and frequency (f) were compared in 23 rats while either awake and unrestrained or anesthetized. FRC was determined from gas compression with closed airway inside a cone-shaped body plethysmograph. In the awake state (mean +/- SD), FRC was 1.02 +/- 0.22 ml/100 g, VT was 0.38 +/- 0.06 ml/100 g, and f was 142 +/- 22 breaths/min. During anesthesia, FRC decreased (P less than 0.01) to 52.9% of awake values, VT increased (P less than 0.01) to 147.4%, and f decreased (P less than 0.01) to 71.8%, leaving minute ventilation almost unchanged. An additional seven rats were used to examine postural effects on FRC during anesthesia, and in another seven animals pleural pressure changes were monitored. Dynamic lung compliance (0.80 ml . kg-1 X cmH2O-1) was not altered by anesthesia, but the pressure-volume curve was shifted 6 cmH2O higher. Thoracic compression, followed by a time-dependent effect of volume history, may account for the major change in FRC. The remainder of the decrease in FRC may be due to lower breathing frequency, loss of inspiratory muscle activity, and/or less airway resistance after anesthesia. Peak diaphragmatic electromyogram per unit VT was shown to increase almost linearly with FRC, indicating that diaphragmatic efficiency was decreased as lung volume was elevated. Functional residual capacity (FRC), tidal volume (VT), and frequency (f) were compared in 23 rats while either awake and unrestrained or anesthetized. FRC was determined from gas compression with closed airway inside a cone-shaped body plethysmograph. In the awake state (mean +/- SD), FRC was 1.02 +/- 0.22 ml/100 g, VT was 0.38 +/- 0.06 ml/100 g, and f was 142 +/- 22 breaths/min. During anesthesia, FRC decreased (P less than 0.01) to 52.9% of awake values, VT increased (P less than 0.01) to 147.4%, and f decreased (P less than 0.01) to 71.8%, leaving minute ventilation almost unchanged. An additional seven rats were used to examine postural effects on FRC during anesthesia, and in another seven animals pleural pressure changes were monitored. Dynamic lung compliance (0.80 ml . kg-1 X cmH2O-1) was not altered by anesthesia, but the pressure-volume curve was shifted 6 cmH2O higher. Thoracic compression, followed by a time-dependent effect of volume history, may account for the major change in FRC. The remainder of the decrease in FRC may be due to lower breathing frequency, loss of inspiratory muscle activity, and/or less airway resistance after anesthesia. Peak diaphragmatic electromyogram per unit VT was shown to increase almost linearly with FRC, indicating that diaphragmatic efficiency was decreased as lung volume was elevated.