The efficacy of picumast dihydrochloride (3,4-dimethyl-7-[4-(4-chlorobenzyl)piperazine-1-yl]propoxycoumarine++ + dihydrochloride) in a dosage of 2 mg o.d. (loading dose = 2 b.i.d. over the first three days) vs. placebo after nasal challenge was investigated in 20 patients with allergic rhinitis in a double-blind, controlled crossover study. The primary objective was the decrease in determinable nasal flow as a result of obstruction caused by swelling of the mucous membrane of the nose following nasal challenge with grass pollen. In addition, picumast dihydrochloride's effect on subjective symptoms elicited by nasal challenge, such as "runny nose" and "irritation", was also assessed. Picumast dihydrochloride's ability to inhibit nasal obstruction after challenge was significantly better than that of placebo in statistical terms. Nasal secretion after challenge with grass pollen was far less pronounced with picumast dihydrochloride than placebo. A positive effect on irritation was not seen after nasal challenge. The results of this study show that picumast dihydrochloride inhibits nasal obstruction and secretion in patients allergic to grass pollens following allergen challenge. It is also expected that picumast dihydrochloride will be able to clearly reduce allergic nasal reactions during times of natural, seasonally-high pollen counts.