Abstract: The drug-receptor interactions of dimethyltrilobine iodide (DTI) with toad rectus abdominis N₂ cholinoreceptor were studied and compared with those of curare. The DTI antagonism of ACh induced muscle contractions was completely overcome by increasing the concentration of ACh and the dose-cumulative-effect curves were parallel. The pA₂ values of DTI and curare were 5.89 and 5.73 respectively. The K_B's of DTI and curare were 2.90×10^-6 mol and 2.84×10^-6 mol respectively. When the dose ratios log(X-1) were plotted versus negative log molar values of DTI, the regression slope was -0.92 which is close to the theoretical value -1.0 of competitive antagonist. These results suggest that DTI is a competitive antagonist.