The blocking action and selectivity of 1 - (? -naphthylmethyl)-2 - methyl - 6,7 - dimethoxy -1,2,3,4 -tetrahydroisoquinoline (86040) on a - a-drenoceptor have been investigated in isolated tissues. 86040 suppressed the inhibition of clonidine for the electrically stimulated twitch response of rat vas deferens ( - adre - noceptor), with pA2 value of 5. 85. 86040 could competitively inhibit anococcygeus muscle contraction (?1 -a-drenoceptor ) induced by phenylephrine with pA2 value of 7. 62. 86040 was found that the selectivity radio to block ?2 - and ?2 adrenoceptor(?1/?2)was 60. these results indicate that 86040 is a potentant relatively selective ?1 - adrenoceptor blocker.