Abstract: The effects of 36 tetrahydroisoquinolines on ³HQNB binding to the M-cholinergic receptors of rat brain were investigated using receptor binding assay in vitro, l-Tetrandrine was shown to have very high affinity to the receptor with K_i value of 7.3 × 10^-8 mol/L. The affinity of d-berbamine, tetrahydrocoptisine and B₁, a synthetic protoberberine derivative, was relatively high with K_i values of 1.9×10^-7, 6.8×10^-7 and 8.1×10^-7 mol/L respectively. Other protoberberine derivatives like B₂, B₃, B₄, B₅, B₆, berbamine derivative E₁ and dauricine derivative D₁ appeared to have moderate affinity and their K_i values fell into the range of 1～2 × 10^(-6) mol/L. No affinity was shown by l-capaurine, l-corydaline, l-cyclanoline, l-stepholidine, dl-tetrahydrojatrorrhizine, d-tetrahydropalmatine, l-tetrahydropalmatine. Thereis enough evidence to support the idea that there is a structure-activity relationship among the compounds tested. It can be inferred that some pharmacological effects of tetrahydroisoquinolines are probably related to M-cholinergic receptors.