Abstract: Amidopyrine is oxidized to 4-aminoantipyrine, which is coupled oxidatively with phenol to form a red product; ammoniacal buffer medium of pH 7.5 to 8.5 and K3Fe(CN)6 as oxidant are used. The qualitative test is sensitive (~1 ppm), and no other common drug reacts in this way. Spectrophotometric measurement (at 510 nm) gives a rectilinear calibration graph for 0.04 to 0.4 mM amidopyrine. In the determination of amidopyrine in different medicaments, the coefficient of variation were <0.5%. Other methods of determination, viz, that involving oxidation by vanadate or titration of the base in acetic acid medium, give less reliable results. Up to 120 samples containing 10 to 100 ppm of 4-aminoantipyrine can be analyzed in 1 h by flow injection analysis based on the described reaction.