2-[2-(1,4-Benzodioxanyl)]-2-imidazoline or a non-toxic salt thereof, substantially free of 2-[2-(2-methyl-1,3-benzodioxyl)]- 2-imidazoline or a non-toxic salt thereof.2-[2-(1,4-Benzodioxanyl)]-2-imidazoline or a non-toxic salt thereof characterised in that the nuclear magnetic resonance spectrum of the compound in a protonated form exhibits multiplets in the region of .tau.4.4 and .tau.5.4.Process for the preparation of 2-[2-(1,4-benzodioxanyl)]-2-imidazoline, pharmaceutical compositions thereof or of its salts, and their use as presynaptic .alpha..sub.2 -adrenoreceptor antagonists.