Indomethacin is a non-narcotic and non-steroidal anti-inflammatory drug used in the treatment of various inflammatory diseases such as rheumatoid arthritis, osteoarthritis, and ankylosing spondylitis. In neonates, it is also used for induction of closure of patent ductus arteriosus (PDA). Its mechanism of action is believed to be through the inhibition of cyclooxygenase. Due to narrow therapeutic window and number of side effects, it's monitoring, particularly in neonates, is recommended. In the gas chromatography method described here, the drug is extracted from serum or plasma using methylene chloride and phosphate buffer (pH 6). The methylene chloride phase containing drug is separated and dried under stream of nitrogen. The drug is derivatized using Bis-(Trimethylsilyl)trifluoroacetamide (BSTFA) with 1% TMCS (trimethylchlorosilane). The derivatized drug is analyzed using gas chromatography-mass spectrometry. Quantitation of the drug in a sample is achieved by comparing responses of the unknown sample to the responses of the calibrators using selected ion monitoring. Meclofenamic acid is used as an internal standard.