We examined the reliability of whole-body plethysmography (WBP) for determining the ventilatory volume in mice. The degree of agreement of tidal volume (VT) determined consecutively by WBP with that by direct plethysmography (DP) or pneumotachography (PT) was analyzed using urethane-anesthetized young male 129/Sv mice. Between WBP and DP, the mean difference of VT was 0.000 ml with a 95% confidence interval of -0.003 to 0.002, whereas that between WBP and PT was 0.000 ml (-0.004 to 0.003). The limits of agreement, or mean +/- 2SD of the difference, were -0.008 and 0.008 ml in WBP vs. DP, and -0.011 and 0.010 ml in WBP vs. PT. These values were 5.1-6.6% of the average VT (0.157 ml in WBP and DP, and 0.165 ml in WBP and PT). Thus, VT can be determined by WBP without any bias and with a systemic error of less than 7% of that measured by the other direct methods examined. Through applying WBP to conscious 129/Sv and 129/SvJ-ICR hybrid mice, we further studied the effects of differences in mouse strain, age and sex on respiratory rate (f) and minute volume (V = VT.f), and their reflex changes in responses to systemic hypoxia and hypercapnia. V was significantly greater in young (10-16 weeks old) male 129/Sv than in young male 129/SvJ-ICR hybrids (139 +/- 11 and 73 +/- 8 ml/min/100 g, respectively), while the pattern of reflex responses was similar. In aged (32-44 weeks old) male 129/Sv, the basal level of V and its reflex increases were all diminished as compared to those in their young counter parts. However, we were unable to find significant differences between aged male and female 129/Sv mice. In conclusion, WBP is a useful and reliable method to determine ventilatory parameters in conscious and anesthetized mice. Furthermore, there are considerable strain- and age-related differences in baseline ventilation and reflex responses to hypoxia and hypercapnia.