The distr. of COHb, O2Hb and reduced Hb and their related gas tensions were detd. in the blood of subjects in equilibrium at various pressure-altitudes with atmospheres containing low conc. of CO. The "laws" of Haldane, derived in vitro, were found to apply to the gas equilibria obtaining in vivo. The value of the relative affinity constant of Hb for CO was 204 [plus or minus]10%. The total barometric pressure played no role in the distr. of CO and O2 at equilibrium. A simple formula was derived from the Haldane equation and confirmed exptlly. to predict the equilibrium values of COHb at altitude by using standard values for blood gas tensions and saturations. The symptoms produced by CO are proportional not only to the blood conc. of COHb but also to the duration of exposure.