We studied ventilatory responses to inspiratory flow-resistive loads in six trained dogs, during quiet wakefulness and non-rapid-eye-movement (NREM) sleep. During studies dogs lay quietly in a lateral position and breathed through an endotracheal tube inserted via a chronic tracheostomy. Linear resistances of 6, 10, 19, and 31 cmH2O X l-1 X s were applied during inspiration for only a single breath to assess the immediate ventilatory response. The highest resistance was also applied for five successive breaths to assess the progressive ventilatory response. Ventilatory responses to hyperoxic progressive hypercapnia were also examined, with and without flow-resistive loading. During loading the maintenance of constant states of quiet wakefulness and NREM sleep was confirmed by electroencephalographic monitoring. Ventilation decreased on the first loaded breath and returned to control in a stepwise manner by the fifth loaded breath. No state-related differences were observed in either the immediate or progressive ventilatory responses. During CO2 rebreathing, the slope of the ventilatory response to CO2 was reduced by loading, with the reduction in slope (as percent of control) greater in the NREM state. We therefore conclude that in the dog immediate and progressive ventilatory responses to resistive loads are unaffected by NREM sleep, whereas the decrease in ventilatory response to CO2 resulting from loading tends to be greater in NREM sleep than in quiet wakefulness.