Thirty patients suffering from perennial allergic rhinitis took part in a cross-over, double-blind study during which they were treated for 3 two-week periods with either 10 mg cetirizine nocte or 60 mg terfenadine given morning and evening, or with a placebo. Both the active products were found to significantly improve the symptoms when compared to placebo (p less than or equal to 0.001). Cetirizine helped to reduce the symptoms of a blocked nose to a significantly greater extent than terfenadine (p less than or equal to 0.05). A significantly larger number of patients (p less than or equal to 0.05) preferred cetirizine (17/30) to terfenadine (6/30). Four patients taking cetirizine, and six of those taking terfenadine, reported a mild sedative effect.