To determine the relative efficacy, to compare the incidence of adverse experiences, and to assess the systemic glucocorticoid effect of nasal preparations of budesonide, 200 micrograms bid, and placebo, 50 adult patients with seasonal allergic rhinitis due to grass pollen were studied in a stratified, double-blind parallel group design. After a 2-week baseline period, budesonide nasal spray, 100 microgram per nostril twice a day, was compared with placebo nasal spray over a 4-week treatment period. Supplementary treatment with chlorpheniramine, 4-mg tablets, was permitted when necessary to control symptoms. Daily symptom and medication diaries were kept by the patients. Investigator assessments of symptoms and side effects were made at clinic visits at 2-week intervals. At baseline and again towards the end of the study, blood samples were drawn for the determination of plasma cortisol levels and 24-hour urine samples collected for the measurement of 17-hydroxycorticosteroid output. Of the 24 men and 26 women entering, 49 completed the study. Symptom scores for sneezing, stuffy nose, and nasal secretion all decreased dramatically from baseline when budesonide treatment was started. The decrease in symptoms was greater for budesonide than for placebo (P < .001). There was no difference between budesonide and placebo with regard to eye itch and rescue medication used. Morning nasal washes were taken during the grass season before treatment was started and 16 to 17 days after. They showed a significant decrease in TAME esterase levels in secretions in the budesonide treated patients (P = .03) but not in the placebo-treated patients.(ABSTRACT TRUNCATED AT 250 WORDS)