A study has been undertaken to determine the accuracy of measuring oxygen saturation and oxygen content of blood with five different, commercially available systems. The indications for using them in medicine are discussed. In animal experiments simultaneous measurements were carried out with variations of oxygen saturation from 7% to 100%, of oxygen content from 0.1 vol% to 18 vol% and hemoglobin content from 5.5 g% to 18 g%. The Lex-O2-Con, the CO-Oximeter IL 182 and the Hemoximeter OSM2 are usable over the total range of variations, but the results of the AO-Oximeter are only correct in the range above 50% oxygen saturation. The information directly displayed by the In Vivo-Hämoreflektometer IVH3 are satisfactory, but its use obviously must be reserved for special situations which warrant the use of the catheter.