1 The definition of PO2 and its relationship to the oxygen saturation (SO2) by the oxygen dissociation curve (ODC) is described with details of the ligands of the ODC and the effects of haemoglobinopathies on P50 (the position of the ODC) and the slope of the ODC (Hill's 'n'). 2 The definition of PCO2 and description of CO2 transport by blood leads to consideration of the basis of acid base balance. 3 Acid base balance is expressed in terms of arterial blood (H+) (or pH) PCO2 relationship using a non‐logarithmic diagram. 4 The measurement of arterial blood gas tensions is described with comments on arterial puncture and available modern automated blood gas electrodes and their calibration. 5 Non‐invasive indirect measurements of blood gas tensions, by ear oximetry are described, with calibration figures on the Hewlett‐Packard 47021A ear oximeter. End‐tidal PO2 and PCO2 measurements by mass spectrometer or infra‐red CO2 analyser, and of transcutaneous PO2 measurements by a heated polarographic electrode are described. 6 These measurements are necessary to study the effects of pharmacological agents on the chemical control of breathing, on pulmonary gas exchange, and on acid base balance in humans. The measurements are in everyday use in clinical practice, both to aid diagnosis of respiratory diseases, assess their severity, and to quantiate the effects of therapeutic agents.