A combination of sequential Injection Analysis (SIA) system and the ion-selective electrode (ISE) detector is considered to be one of a very effective analytical method. Reaction of lactate with lactate oxidase produces pyruvic acid and hydrogen peroxide. When the produced hydrogen peroxide is reacted with 4-aminoantipyrine (4-AAP) and 4-fluorophenol (4-FP) in the presence of horseradish peroxidase (HRP), F⁻ ion is produced. The concentration of lactate can be indirectly determined by measuring the F⁻ ion by F⁻ ISE. Based on this principle of detection of lactate, a new measurement method of lactate using an SIA with ISE detector was developed.