Spectrophotometric methods for determination of N-Acetyl-p-aminophenol (NAAP) in pharmaceutical preparations are described. NAAP was hydrolysed by refluxing with HCl to produce p-aminophenol which reacts with Na₃ [Fe (CN)₅NH₃] in alkaline solution to give a blue-green color (λ_max : 700 mμ), or condenses with phenol in alkaline solution to produce a blue indophenol dye (λ_max : 620 mμ). NAAP in pharmaceutical preparations could be determined by these methods within a standard deviation of ±0.6%. Influences of about forty compounds, which are used with NAAP, were examined. Sulpyrine interferes this method but its interference can be eliminated by using the standard solution containing the same amount of sulpyrine.