Post- and presynaptic α-adrenoceptors in the canine mesenteric artery show distinct features of α<sub>1</sub> and α<sub>2</sub> types, respectively. The effects of a newly synthesized pressor agent, 2-[(5-chloro-2-methoxyphenyl)azo]-1H-imidazole (M6434), on the pre- and postsynaptic α-adrenoceptors of the canine mesenteric artery were examined pharmacologically. M6434 produced a contractile response in the mesenteric artery. The EC<sub>5</sub>o value of M6434 was 1.1 × 10<sup>–7</sup> M, that was almost the same as that of norepinephrine but significantly lower than the value seen with methoxamine. The maximum amplitude of contraction developed by M6434 was approximately 81 % of that induced by norepinephrine. The contractile response to M6434 was competitively inhibited by prazosin (pA<sub>2</sub> = 9.23 ± 0.08) and yohimbine (pA<sub>2</sub> = 7.85 ± 0.09). M6434 also reduced the <sup>3</sup>H-efflux evoked by electrical transmural stimulation in the arterial tissues preincubated with <sup>3</sup>H-norepinephrine, and the maximum inhibition corresponded to about 60 % of the case of norepinephrine. This presynaptic effect was inhibited by yohimbine but not by prazosin. The results show that M6434 is almost a full agonist for α<sub>1</sub>-adrenoceptor with partial α<sub>2</sub>-effect.