The antihistaminic properties of the tricyclic antidepressants have been recognized since these compounds were first developed. Antidepressants, which are equally effective for treating depression or used in the treatment of chronic urticaria, have varying in vitro antihistaminic properties. We compared the duration of H1-receptor blockade by two tricyclic antidepressants, doxepin (the most potent antihistamine) and desipramine (the least potent antihistamine), in a single dose, double-blind, noncrossover study. After baseline prick test with histamine phosphate 1:1000 by Multitest (Lincoln Diagnostics, Decatur, Ill.), the suppression of cutaneous histamine-induced wheal-and-flare responses were measured daily for 7 days in 33 healthy volunteers who were randomly administered a single 25 mg dose of oral desipramine or doxepin. Significant differences in the suppression of the wheal-and-flare responses to histamine between the two drugs were noted (p less than 0.05) during the first 3 days. Desipramine suppressed the wheal for 2 days and flare for 1 day. Doxepin suppressed the wheal for 4 days and flare for 6 days. Our results suggest doxepin should be withheld for at least 7 days before allergy skin testing.