The non-sedating, second-generation H1-receptor antagonists such as terfenadine, astemizole, loratadine and cetirizine differ considerably from each other in their pharmacokinetics and pharmacodynamics. They are generally well absorbed when administered orally. They have extremely variable serum elimination half-life values. The maximum antihistaminic effect of these medications occurs several hours later than peak serum concentrations do. The duration of the antihistaminic effect is much longer than would be predicted from the serum elimination half-life values. The relative incidence of anticholinergic and central nervous system adverse effects caused by these medications is similar to that produced by placebo. The introduction of the second-generation H1-receptor antagonists represents a major advance in symptomatic therapy of allergic disorders such as allergic rhinitis and urticaria.