The test - retest reliability coefficients of the values of measurements made on the pH, the Pco₂, the standard bicarbonate, the base excess, and the buffer base, of arterial whole blood were estimated for a group of 30 male subjects with the use of the Astrup Micro-equipment, the Siggaard-Andersen revised Nomogram, and a testing program on three successive mornings. Reliability coefficients were estimated for these parameters in both the resting and the post-exercise conditions, and the

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... ndivldual and the inter-individual variances were estimated for each reliability coefficient. It was found that the standard bicarbonate and the base excess had the more reliable values, and that the pH and PC0₂ had less reliable values. It was shown that the values of measurements of pH, standard bicarbonate and base excess were significantly different on Day 1 from those values on Days 2 and 3. This effect was attributed to apprehension towards the testing experience and to the strangeness of the testing environment. The measurement errors of the Astrup Micro-equipment and the Siggaard-Andersen revised Nomogram were estimated for the individual parameters. It was found that the buffer base was the only parameter in which the pH meter measurement error variance was large enough to have a decided effect upon the value of the reliability coefficient. The Nomogram measurement error variances were so small that they could be considered negligible. The temporal measurement error of the collection of blood - the error inherent in the time difference between the collection of successive tubes of blood - was investigated for the pH parameter. This temporal measurement error was shown to be practically negligible although statistically significant in the resting condition, but was shown to have both practical importance and statistical significance in the post-exercise condition.