A sensitive, specific, and quantitative GLC method for the determination of phendimetrazine in serum is described. The procedure involves the addition of an internal standard to serum samples, followed by extraction at pH 13 into toluene. The extracted bases are back-extracted into 1 ml of 4 M hydrochloric acid and again into 100 mu1 of chloroform after making the 4 M hydrochloric acid extract basic with 1.5 ml of 4 M sodium hydroxide. The sensitivity of the method is such that 25 ng of material can be detected in 5 ml of serum.