A specific gas-liquid Chromatographic method for the determination of nanogram amounts of morphine in plasma and cerebrospinal fluid is described. The procedure requires conversion of morphine, after extraction and concentration from the biological fluid, to its trimethylsilyl ether derivative. Bis(trimethylsilyl)trifluoroacetamide plus 1 % trimethylchlorosilane is used both as the solvent and silylating agent in the derivatization. Extraction and conversion are linear over the range 25-500 ng/sample with the limit of sensitivity of the method being about 25 ng/sample. © 1969.