The effects of body position on ventilatory responses to chemical stimuli have rarely been studied in experimental animals, despite evidence that position may be a factor in respiratory results. The purpose of this study was to test whether body position could affect acute ventilatory responses to 4-min periods of moderate hypercapnia (5% CO 2 in O 2 ) and poikilocapnic hypoxia (15% O 2 in N 2 ) in the urethane-anaesthetised mouse. Respiratory measurements were conducted with mice in the prone and supine positions with a whole-body, single-chamber plethysmograph. During hypoxia, the time course of minute ventilation (V˙ E ) was similar in the two positions, but the breathing pattern was different. After the response peak, V˙ E depended on respiratory frequency (f) and tidal volume (V T ) in the prone position but mainly on V T in the supine position. In the supine position, f declined below the baseline values toward the end of hypoxic exposure. During hypercapnia, there were no ventilatory differences between the prone and supine positions. Brief hypoxic exposure elicited f depression in the supine position in the anaesthetised mouse. The depressive effect on f suggests that the supine position may not be optimal for sustaining ventilation, particularly during hypoxia.