Ans: It is generally believed that a₂-agonists require both ortho substituents to force a non-planar structure for optimal binding to the a₂ receptors (i.e., the aromatic ring and the five-membered heterocyclic ring should be perpendicular to each other). On the other hand, binding to the a₁ receptors requires only slightly twist rings conformation. Thus, any imidazoline-type agonists with only one such ortho substituent are a₁ agoinsts. Xylometazoline and oxymetazoline, cannot bind a₂ receptors due to the bulky para-substituent. Thus, it has only a₁-agonist activity.