The responses of the diaphragm, external oblique, and transversus abdominis muscles to hyperoxic hypercapnia and isocapnic hypoxia were studied in four awake dogs to test the hypothesis that central and peripheral chemoreceptor inputs result in different patterns of respiratory muscle activation. The dogs were trained to lie quietly in place, and electromyographic (EMG) discharges of the diaphragm (EMGdi), external oblique (EMGeo), and transversus abdominis (EMGta) were recorded from chronically implanted electrodes. Both hypercapnia and hypoxia recruited EMGeo and EMGta activity, but at comparable levels of minute volume of ventilation the EMG activity of the abdominal muscles was greater during hypercapnia than during hypoxia. However the two chemical stimuli also resulted in different tidal volume (VT) and respiratory frequency responses at any given minute volume of ventilation. When EMG activity was reanalyzed as a function of VT, EMGeo and EMGta were the same for a given VT whether induced by hypercapnia or hypoxia, but EMGdi was consistently greater during hypoxia than during hypercapnia. When the vagus nerves were blocked by cooling exteriorized cervical vagal loops, all abdominal muscle EMG activity was abolished. The findings support the concept that stimulation of the central and peripheral chemoreceptors results in asymmetric activation of the inspiratory and expiratory respiratory muscles. The findings also indicate that afferent vagal stimuli play an important facilitatory role in activation of the abdominal expiratory muscles.