BACKGROUND: Cetirizine, a new once-daily highly specific H₁-antagonist, has been shown in conventional studies to be efficacious in the treatment of seasonal and perennial allergic rhinitis and chronic idiopathic urticaria. OBJECTIVE: The efficacy, duration and onset of action, and safety of cetirizine, 10 mg once daily, was compared with that of loratadine, 10 mg once daily, and placebo in a field study of patients with seasonal allergic rhinitis. METHODS: This was a randomized, double-blind, parallel, double-dummy study conducted over 2 days in spring allergy season at outdoor parks in San Diego and Iowa City. Study medication was administered at 10:00 AM on both days. After screening, eligible patients completed rhinitis symptom diaries in the park hourly from 7:30 to 9:30 AM (baseline); at 10:30 AM and hourly from 11:00 AM to 4:00 PM (period I); at 6:00, 8:00, and 10:00 PM at home (period II); and the next day in the park hourly from 8:00 to 10:00 AM (period III), and from 11:00 AM to 4:00 PM (period IV). Major and total symptom complex scores, global efficacy and overall satisfaction, and adverse events were assessed. RESULTS: Of the 279 patients (140 men and 139 women; mean age, 29 years) randomized to treatment, 278 were included in the efficacy analysis. Cetirizine produced significantly greater mean reductions than loratadine or placebo in major symptom complex severity scores at all periods (p ≤ 0.05), except period I for placebo. Cetirizine also produced mean reductions in total symptom complex severity scores that were superior to loratadine at every evaluation period (p < 0.05) and were statistically different from placebo at period II (p < 0.01). A rapid onset of action was observed with cetirizine, as was a better response pattern in the patient global assessment of efficacy compared with loratadine. Study medications were well tolerated; no patient stopped treatment because of side effects. The incidence of somnolence with cetirizine was 13% versus 2% with placebo (p < 0.05); headache occurred more frequently with loratadine (23%) than with cetirizine (11%, p = 0.03). CONCLUSION: Cetirizine relieved rhinitis symptoms more effectively and quickly than loratadine and placebo in this field study of seasonal allergic rhinitis. Both active agents were generally well tolerated. (J ALLERGY CLIN IMMUNOL 1996;97:617-26.)