Human venous blood pH, P(CO(2)) and P(O(2)) were measured with I. L. Meter. Its reproducibility and response rate were evaluated as well as methods of the procedures. 1) Time required to reach the stability of pH, P(CO(2)), and P(O(2)) reading after the sampie injection was 1～1.5 min., 2～2.5 min. and 45～75 sec., respectively. It is recommended in the measurement of these parameters at the same time that the sample injection starts with P(CO(2)) electrode, followed by pH and P(O(2)) in this order and that readings are recorded in the order of pH,
P(O(2)) and P(CO(2)). 2) Range of differences between the two values measured in the interval of 3～5 min. were pH : -0.010～0.020 (mean: 0.003), P(CO(2)) : -1.0～1.0mmHg (mean: 0.4) and P(O(2)) : -1.0～0.0mmHg (mean: -0.5). Their 5% rejection limits were 0.021≧x(o)≧-0.015, 2.0≧x(o)≧-1.2mmHg and 0.3≧x(o)≧-1.3mmHg, respectively. 3) The pH, P(CO(2)) and P(O(2)) of the heparinized venous blood stored in ice water showed no significant changes in 60 min. and they gave practically the same results as the measurement just after shedding.