We examined changes in O2 uptake, CO2 output, blood pressure and heart rate following tourniquet deflation in 23 patients undergoing orthopaedic surgery of the lower extremities. A pneumatic tourniquet was applied for periods ranging from 21 to 106 min (mean 51 min). Prerelease values of VO2 (O2 uptake at each min) and VCO2 (CO2 output at each min) were 201 (37) and 174 (38) (mean (SD)) ml.min-1, respectively. Significantly, VO2 and VCO2 increased by 55% and 80%, respectively, at 2 min after tourniquet release and returned to prerelease values within 8 min. The blood pressure fell significantly and the heart rate rose significantly. The increases in CO2 output and O2 uptake were dependent on the length of tourniquet inflation time; Y = 4.7 x (tourniquet time) + 54, r = 0.88, (p less than 0.001) for CO2, and Y = 1.3 x (tourniquet time) + 99, r = 0.52, (p less than 0.05) for O2. The slope of the increase in CO2 output as a function in inflation time was 3.6 times greater than that of O2 uptake. In conclusion, CO2 output and O2 uptake increased transiently after tourniquet deflation and the extent of the increase in CO2 output is more than threefold as compared with that in O2 uptake.