Sequential MR examinations of the nasal cavity and paranasal sinuses were performed within a 6-8 h period in five normal volunteers. The nasal mucosal volume and signal intensities (T2 weighted) were shown to alternate from one side to the other during this period. When a topical vasoconstrictor was applied, this cycle was interrupted. These cyclical changes were limited to the mucosa of the turbinates, nasal cavity, and ethmoid sinus, representative of the nasal cycle. The maxillary, frontal, and sphenoid sinuses were not affected. The hyperintensity of the nasal cycle on T2-weighted images was similar to that shown by inflammatory mucosa. Squamous cell carcinoma within the nasal cavity and paranasal sinuses has characteristically demonstrated a lower signal intensity on T2-weighted images. Awareness of these cyclical nasal and paranasal appearances reduces the likelihood of inflammatory disease being confused with normal physiologic changes within the nasal area.