Acetylation with acetic anhydride, under both aqueous and anhydrous conditions, has been utilized to derivatize various biogenic amines and psychotropic drugs for subsequent analysis by gas chromatography (GC) or gas chromatography-mass spectrometry (GC-MS). Under basic aqueous conditions, acetic anhydride derivatizes phenols and amines but not alcohols; under anhydrous conditions, all three functions are acetylated. Primary amines, once derivatized with acetic anhydride, can be further derivatized with other reagents; these diderivatives have proven useful for subsequent analysis by GC or GC-MS. Examples of applications of derivatization with acetic anhydride to analysis of biogenic amines, antidepressants, antipsychotics, and some of their metabolites are presented.