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We hypothesized that heart rate variability (HRV) can be used as a physiological monitor of exposures to hypercapnia. HRV was analyzed in 15 male subjects breathing air and then air plus 4%, 5% or 6% carbon dioxide (CO2). A progressive elevation in randomness of HRV best depicted as a divergence of data on Poincaré plots of self-similarity occurred as CO2 increased, and resolved when returning to ambient conditions. Results indicate that HRV may be useful as a prompt (< 2 minute) early warning of hypercapnia that occurs prior to overt symptomatology.