We have recently reported the synthesis and alpha 2-antagonist activity of the methoxy derivative 2 [2-(2-methoxy-1,4-benzodioxan-2-yl)-2-imidazoline] and described the enhanced potency of this compound over the parent 1,4-benzodioxan, idazoxan, in reversing the inhibition caused by alpha 2-adrenoreceptor agonists of the electrically induced twitch in the rat or mouse vas deferens. It was of interest to us to discover whether a similar substitution in the structurally related alpha 2-adrenoreceptor antagonists piperoxan, prosympal, and fenmetazole would similarly enhance potency. We subsequently discovered that this was not so and potency was decreased markedly. In particular, that of the methoxy derivative of piperoxan was ca. 220 times less than the parent structure.