Chlorpheniramine is widely used, but there is very little information about its pharmacokinetics and efficacy in children. In 11 patients with allergic rhinitis, ages 6 to 16 yr, we found a mean serum chlorpheniramine half-life of 13.1 ± 6.3 hr, a mean volume of distribution of 7.0 ± 2.8 L/kg, and a mean clearance rate of 7.2 ± 3.2 ml/min/kg. Suppression of symptoms and signs of allergic rhinitis and suppression of the histamine-induced wheal and flare responses occurred when mean serum chlorpheniramine concentrations ranged from 2.3 to 12.1 and 4.1 to 10.0 ng/ml, respectively. This, combined with the large volume of distribution for the drug, suggests that tissue binding is an important aspect of chlorpheniramine pharmacokinetics.