Oxatomide is an H1 receptor antagonist, which also inhibits antigen-induced mediator release from sensitized mast cells and basophils. The effects of ingested oxatomide (30 mg/capsule) were compared with those of chlorpheniramine (5 mg/capsule) in a double-blind non-crossover study in two groups of twenty-five subjects with ragweed pollen-induced hay fever during the pollen season. Oxatomide or chlorpheniramine was started when symptoms became troublesome and was used in a dose of two to four capsules daily according to severity. Other medications were added, in a standardized way, if hay fever symptoms were not controlled by the test capsules. Forty-one subjects completed the study; nineteen received oxatomide and twenty-two chlorpheniramine. Mean symptom scores, number of test capsules and number of doses of additional medication were not significantly different between the two groups. Drowsiness was reported by subjects in both groups but was worse in the chlorpheniramine group (P less than 0.05). We conclude that, in the studied doses, oxatomide is as effective for the treatment of allergic rhinoconjunctivitis as chlorpheniramine and produces less drowsiness.