Plasma levels of opioid drugs, Mr 1268 MS (3-(2-methyl-3-furylmethyl)-1,2,3,4,5,6-hexahydro-6 alpha,11 alpha-dimetyl-2,6-methano-3-benzazocin-8-ol methane sulfonate), Mr 2033 CL ( [2 alpha, 3(S*), 6 alpha, 11 R*]-(+/-)-1,2,3,4,5,6-hexahydro-6,11-dimethyl-3-[(tetrahydro-2-furanyl) methyl]-2,6-methano-3-benzazocin-8-ol hydrochloride), MRZ 2549 BR ( (2-methoxypropyl)-1,2,3,4,5,6-hexahydro-6 alpha, 11 alpha-dimethyl-2,6-methano-3-benzazocin-8-ol hydrobromide), and pentazocine, have been determined by a radioreceptor assay after enteral and parenteral administration to human volunteers as well as to laboratory animals (monkeys and dogs), using [3H]dihydromorphine as radioligand and freshly prepared membrane homogenates of rat brains. The concentration of parent compound drug has been evaluated after a specific extraction from plasma. The method described presents a universal possibility for the determination of opioid drugs of different structures in plasma and other body fluids.