Abstract: An improved cuvette unit suitable for use in blood oxygen analyzing and plotting apparatus in accordance with U.S. Pat. No. 4,304,488. A cuvette cavity, preferably cylindrical, is formed in a transparent plastic block. A Clark electrode is embedded in said block and arranged to project into the interior of the cavity. A horizontally-directed gas inlet is introduced adjacent the bottom of the cavity, for introducing the gas (oxygen, or an inert gas) and also providing efficient agitation. Among the benefits are the avoidance of mechanical agitation elements and the speeding up of the oxygen association and dissociation tests.