A spectrophotometric method for determination of N-Acetyl-p-aminophenol (NAAP) in pharmaceutical preparations is described. NAAP was oxidized by HNO2 in acidic solution to give an yellow color which was changed to orange-yellow by addition of strongly alkaline solution and determined spectrophotometrically at 437 mμ. With this method, NAAP in the range 7∼28 μg/ml could be determined within a standard deviation of ±0.77%. Because of its simplicity and rapidity, this method is well suited for analysis of NAAP in complex pharmaceutical preparations of antipyretic analgesics. Influences of about forty compounds, which are used with NAAP, were examined. Aminopyrine and sulpyrine interfer this method, but aminopyrine can be separated by extraction and a sample containing sulpyrine can be successfully determined by extracting with CHCl3 after heating with diluted hydrochloric acid.