We have developed a method capable of determining the presence of pentobarbital residues in dry, extruded dog food at concentrations of 5 ppb to 20 ppb. Dry dog food is ground to a uniform size and shaken overnight in methanol. A portion of the methanolic extract is evaporated and the oily residue is reconstituted in phosphate buffered saline. Sodium acetate buffer is added to the aqueous extract in preparation for solid phase extraction. The pentobarbital is eluted with hexane/ethyl acetate. The extracts are evaporated to dryness under a stream of nitrogen and dissolved in dimethyl sulfoxide and tetramethyl ammonium hydroxide. The pentobarbital is derivatized by the addition of iodomethane. The derivatized sample is acidified with dilute hydrochloric acid and extracted with hexane. The organic layer is removed and evaporated. The remaining residue is reconstituted in a small volume of ethyl acetate for GC-MS analysis. This simple procedure can be used to screen for, confirm the presence of and quantitiate pentobarbital in extracts of dry extruded animal feeds. After separation by GC, The samples are analyzed by electron impact MS using selected ion monitoring of 4 ions diagnostic of pentobarbital.