Changes in the intensity of EMG activity in the costal diaphragm, crural diaphragm and external intercostal muscle during inspiration were assessed in intact, awake lambs following abrupt transitions in the composition of the inspired gases from air to either a hypoxic/hypoxic mixture (10% O2, 90% N2), a hyperoxic/hypercapnic mixture (40% O2, 6% CO2, 54% N2) or a hypoxic/hypercapnic mixture (10% O2, 6% CO2, 84% N2). A regression method was used to compare the dynamic responses of the three muscles over the 10-min period following each transition. The relationship between the dynamic response functions of the costal and crural diaphragm was the same in each of the three experimental conditions, indicating that these separate components of the diaphragm comprise a single functional unit during breathing. The relationship between the dynamic response functions of the external intercostal muscle and the costal diaphragm varied according to the composition of the inspired gas mixture. This result suggests that the central and peripheral chemoreceptors exert differential effects on the activation of the diaphragm and the external intercostal muscles during breathing, consistent with the hypothesis that sensory information from these receptors is processed, at least in part, in parallel pathways which project separately to the phrenic and external intercostal motoneurons.