Histamine is a central nervous system (CNS) neurotransmitter. It acts in the brain via three receptors, H(1), H(2), and H(3). It is a mediator of "wakefulness" and its activity is necessary to maintain wakefulness, alertness, and reaction time. These activities can be impaired by H(1)-antagonists (reverse agonists) capable of penetrating the blood-brain barrier. By blocking the homeostatic effects of histamine in the CNS, drowsiness and functional impairment with or without drowsiness can occur. Several tests have been designed to assess the effects of antihistamines on the CNS. These include subjective measurements of drowsiness and more objective measurements of impairment. Second-generation antihistamines have been designed to minimize blood-brain barrier penetration by reducing lipophilicity and increasing the affinity for P-aminnoglycoprotein.