Transnasal absorption of pharmaceutical drugs has been recognized as an interesting alternative to the more conventional routes of administration. Even though the nasal mucosa can be used for transnasal drug delivery, it still remains an absorption barrier with specific constituants ensuring a protective function against foreign material penetration. The nasal mucosal barrier is organized at three different levels: a physical barrier composed of the epithelial cells and mucus, a temporal barrier controlled by the mucociliary clearance, and an enzymatic barrier acting principally on peptides and proteins. The mechanisms of action of such barriers are reviewed as well as some factors affecting them, and several absorption enhancement possibilities are discussed.