Abstract: The Pellagri's reaction has been known to be a specific and sensitive color reaction for morphine and its derivatives. However, because of the lack of mastering the proper experimental conditions, this reaction always fails to give consistent results and consequently can only be applied as a qualitative but not a quantitative method. According to the following experimental condition, we have succeeded to determine morphine colorimetrically with good reproducibility and to raise up the identification limit to 1 γ morphine base. After evaporating the aqueous solution of morphine hydrochloride or chloroform extract to dryness on a water bath, add 3 drops of HCI-H₂SO₄ (10:1) and heat for 30 minutes. Remove it from the water bath and add 6 drops of pH 7 buffer solution (KH₂PO₄-NaOH), 8 drops of 10% NaHCO₃ solution and 3 drops of 2% HIO₃ solution. Heat for 1/2 minute over water bath, then add 5 ml of water and heat for another 1/2 minute. Extract with 5 ml of n-butyl alcohol, and use the organic layer for the quantitative determination by measuring its extinction at 595 mμ or using a red brown filter instead. For the qualitative purpose, the solution to which HIO₃ has been added, is heated for 1 minute on the water bath, transferred to a test tube (5×70 mm), and is shaken with 1-2 drops of n-butyl alcohol. In the presence of morphine and its derivatives the alcoholic layer should be bluish green.