This 1 week study compared the efficacy of once daily administration of 10 mg loratadine with 120 mg terfenadine in out-patients with seasonal allergic rhinitis. It focussed on the efficacy of treatment at the end of the 24 h period following a daily dose. The study was designed as a double-blind, randomized, parallel-group trial, and 41 patients were enrolled and evaluated for efficacy. Patients took an initial dose at the study site and returned on days 2 and 8. At day 2 (24 h after the initial dose), according to the physician's evaluation 57% of loratadine-treated patients had a good or excellent response, compared to 50% of those given terfenadine. At day 8, 24 h after the final dose, 71% of the loratadine-treated patients and 35% of the terfenadine-treated patients had a good or excellent response (P = 0.03). At days 2 and 8, reductions in mean symptom scores measured 22, 23 and 24 h after the initial and final doses showed an indication of being greater with loratadine than with terfenadine (nonsignificant due to small sample size). The incidence of sedation was similar in both groups. It is concluded that 10 mg loratadine, administered once daily, controls the symptoms of rhinitis more effectively than 120 mg terfenadine given once daily in the last few hours of the 24 h dosing period.