Abstract: AIM　To study the antagonistic effect of 1-(2,6-dimethylphenox)-2-(3,4-dimethylphenyl ethylamino) propane hydrochloride (DDPH) on α₁-adrenoceptor (AR). METHODS　Radioligand binding assay was used. Specific ¹²⁵I-BE2254 (2-β(4-hydroxyphenyl)-ethyl aminomethyl-tetralone) binding was measured by incubating membrane of rat cerebral cortex, spleen and the three cloned α₁-AR subtypes (α_1A, α_1B, α_1D) stably expressed in human embryonic kidney 293 cell preparation with a single concentration of ¹²⁵I-BE2254 in the presence of 14 concentrations of DDPH. Equilibrium binding constant (K_I) and Hill coefficients (ⁿH) were determined from Hill plots. The -log value of the K_I was expressed as pK_I. Contractile responses of isolated rat aorta, renal artery ring and spleen were determined. The pA₂ values for DDPH in competitively inhibiting NE-stimulated contraction of tissues were measured with the method of Ainlakshana and Schild. RESULTS DDPH competitively inhibited binding of 125I-BE2254 to α₁-AR in a concentration-dependent manner. The pK_I values for DDPH in rat cerebral cortex and spleen were 7.17±0.06 and 7.41±0.11, respectively, and the Hill efficiency values were not significantly different from unit. The pK_I values for cloned α_1A, α_1B and α_1D-AR were 7.21±0.12, 6.88±0.04 and 7.26±0.06, respectively, and the Hill efficiency values were not significantly different from unit. Contractile studies showed that DDPH competitively antagonized the NE concentration-response curve with a pA2 values of 7.40±0.23 in aorta, 7.41±0.04 in renal artery and 7.63±0.07 in spleen and the slopes of schild plot were not significantly different from unit. The pK_I values for DDPH in tissues and the cloned α_1A or α_1D-AR were shown to fit well in with the pA2 values in antagonizing NE-induced constriction in rat isolated aorta, renal artery and spleen. CONCLUSION　These results suggest that DDPH appear to be a non-subtype selective competitive antagonist for α1-AR.