An enhanced and sensitive GC–ECD assay is presented for the highly specific angiotensin-converting enzyme (ACE) inhibitor, Captopril. This method provides an improved sensitivity over other previously published assays of Captopril derivative. The method involves derivatization with N,N,N',N'-tetramethyl-2-butenediamide and formation of its methyl ester followed by a rapid extraction technique by using acetonitrile as deproteinizing agent. Enalapril was used as internal standard, and XE-60-S-valine-S-phenylethylamide (Chrompack), 25 m x 0.25 mm I.D. and 0.12 pm film thickness was used as the stationary phase in the capillary GC column. Captopril was measured as the methyl ester of its N,N,N',N'-tetramethyl-2-butenediamide derivative. The method was fully validated according to United States Food and Drug Administration requirements (linearity, precision, trueness, quantification limit, detection limit, recovery, stability and specificity). The assay was linear from 0.010 to 2.0 µg/ml with a mean recovery of 99%.