The present investigation examined the relationship between CO2 sensitivity [at rest (SR) and during exercise (SE)] and the ventilatory response to exercise in ten elderly (61-79 years) and ten younger (17-26 years) subjects. The gradient of the relationship between minute ventilation and CO2 production (delta VE/delta VCO2) of the elderly subjects was greater than that of the younger subjects [mean (SEM); 32.8 (1.6) vs 27.3 (0.4); P < 0.01]. At rest, SR was lower for the elderly than for the younger group [10.77 (1.72) vs 16.95 (2.13) l.min-1 x kPa-1; 1.44 (0.23) vs 2.26 (0.28) l.min-1 x mmHg-1; P < 0.05], but SE was not significantly different between the two groups [17.85 (2.49) vs 19.17 (1.62) l.min-1 x kPa-1; 2.38 (0.33) vs 2.56 (0.21) l.min-1 x mmHg-1]. There were significant correlations between both SR and SE, and delta VE/delta VCO2 (P < 0.05; P < 0.001) for the younger group, bot none for the elderly. The absence of a correlation for the elderly supports the suggestion that delta VE/delta VCO2 is not an appropriate index of the ventilatory response to exercise for elderly humans.