The purpose of this study was to determine if the contribution of a muscle to an increase in VT remained the same regardless of the chemical stimulus used to provoke the increase. In seven anaesthetized cats one phrenic nerve was transected and placed on platinum electrodes. The activity was rectified and integrated. After a control period the cat was given either an 8 percent CO2 in air or an 8-10 percent O2 mixture to breathe for 30-45 breaths until a relatively constant VT appeared. Phrenic nerve activity/time (PE) was correlated with tidal volume (VT) for each breath. The linear correlation coefficient (r) for the hypercapnic response was 0.958; for the hypoxic response it was 0.858. The slope of the hypercapnic response (PE, VT) was 0.0619 while that for hypoxia was 0.0379. The rate of phrenic nerve activity was significantly more responsible for the generation of an increase in VT than was the duration of the phrenic burst. And the relative contribution of each was significantly different for the two different stimuli. These results suggest that when equal VT is demanded in response to hypercapnia or hypoxia, the diaphragm is less active in generating the increase in VT during hypercapnia. Or if the diaphragm contracts equally in generating equal VT's, more neural energy per unit time is required if the stimulus is hypoxia.