A sensitive and specific enzymatic-isotopic method of determining plasma amphetamine concentrations in man is described. The assay is based on the transfer of the tritiated methyl group of S-adenosyl-L-[methyl-³H]methionine to amphetamine in the presence of a partially purified N-methyltransferase from rabbit lung. With this assay as little as 10 nanograms of amphetamine per milliliter of plasma can be accurately determined. The concentrations of d- and 1-amphetamine in the plasma after 20 to 30 milligrams of the drug had been ingested by human subjects are reported.