Both the transient hypoxic ventilatory drive test and the single-breath carbon dioxide (CO2) response test have been used to assess peripheral chemoreflex sensitivity. We tested their comparability in 14 healthy adults (10 men, aged 31-73 years, mean 55.4 years). The within-subject reproducibility of both tests was also assessed (n = 7 for each). The mean transient hypoxic ventilatory response was 0.287 +/- 0.0591 min-1 (%Sao2)-1 (mean +/- SEM, range 0.018- 0.718) and single-breath CO2 response was 0.276 +/- 0.0411 min-1T-1 (range 0.081-0.501). Both tests were reproducible with a mean coefficient of variation of 20.1% and 17.7%, respectively. There was, however, no significant correlation between the results of the transient hypoxic and single-breath CO2 tests when data were compared by linear regression analysis (r = 0.23, P = 0.43), suggesting that separate pathways of the peripheral chemoreflex existed for hypoxia and hypercapnia, respectively, and that these tests were specific for each. The authors conclude that these tests are reproducible but need to be used in combination for an adequate assessment of the peripheral chemoreflex.