During pregnancy aggravation of nasal allergic symptoms is occasionally observed in subjects with nasal allergy. To evaluate the effect of female hormones on the muscarinic receptor and the alpha 1-adrenergic receptor of the nasal mucosa, receptor binding assay was performed using the nasal mucosa of pregnant guinea pigs and of male guinea pigs; the latter were treated either with estrogen or with progesterone. Pregnancy induced a significant decrease of the density of the alpha 1-adrenergic receptor of the nasal mucosa (p less than 0.05). Estrogen induced a significant increase of the density of the muscarinic receptor (p less than 0.05), while progesterone induced a significant decrease of the density of the alpha 1-adrenergic receptor of the nasal mucosa of guinea pigs. When similar changes of the receptor can be induced by female hormones in the nasal mucosa of the human subjects, this may facilitate secondary development of hyperreactive nasal symptoms in subjects who have been sensitized before without apparent nasal symptoms.