This study was undertaken to confirm whether transcutaneous (tc) gas analysis during hemorrhagic shock could be used as an alternative to mixed venous gas analysis. Tc gases were measured and correlated with arterial and mixed venous gases in 10 anesthetized dogs during hemorrhagic shock and volume resuscitation. Throughout this experiment PtcO2 correlated well with PvO2 (r = 0.78, P less than 0.01), while PaO2 remained mostly constant, and PtcCO2 correlated well with PvCO2 (r = 0.82, P less than 0.01) rather than with PaCO2 (r = 0.63, P less than 0.01). A more detailed observation showed that during progressively decreased cardiac output, PtcO2 became lower than PvO2 and PtcCO2 became higher than PvCO2. We inferred from these observations that the changes of tc and mixed venous gases reflected those gases in tissues and that during severe shock maldistribution of peripheral blood flow prevented mixed venous gases from coming into equilibrium with tissue and tc gases. We conclude that the measurement of tc gases during shock is a more convenient and more reliable monitor of tissue gases than mixed venous analysis.