An automatic reaction rate method is described for the determination of lactic acid in blood. The L(+)-lactic acid is selectively oxidized in the presence of lactic dehydrogenase and diphosphopyridine nucleotide to form an absorbing species. The time required for the formation of a fixed amount of DANH is measured automatically and related directly to the lactic acid concentration. The method is sensitive, more rapid than purely chemical procedures, and relative errors are only about 1-3% for the range 6-200 µg of lactic acid. The coefficient of variation for the determination of lactic acid in 0.2 ml of blood is 3%. Measurement times vary from a few seconds to about 2 min.