The effects of three commonly used H1 antihistamines on the nasal vascular and airway resistances were studied in the dog. Promethazine hydrochloride decreased nasal vascular resistance but increased nasal airway resistance in a dose-dependent manner. Diphenylpyraline hydrochloride in low doses increased nasal vascular resistance without affecting much nasal airway resistance while in high doses decreased nasal vascular resistance but increased nasal airway resistance. Chlorpheniramine maleate in low doses increased nasal vascular resistance but decreased nasal airway resistance while in high doses decreased nasal vascular resistance without affecting much nasal airway resistance. It was concluded that different H1 antihistamines might exert vasoconstrictor or vasodilatatory action on both the resistance and capacitance vessels of the nasal vascular bed depending on the type and the dose of the drug used.