The potential use of three 2-hydroxypropyl ether derivatives of cyclodextrins (HP-alpha-, HP-beta- and HP-gamma-CyDs) as biocompatible solubilizers and stabilizers for oleic acid, a lipophilic absorption enhancer, was assessed in the nasal absorption of buserelin, an agonist of luteinizing hormone-releasing hormone, in rats. HP-CyDs increased the aqueous solubility of oleic acid and protected it against oxidation through the formation of inclusion complexes with the efficacy increasing in the order: HP-gamma-CyD << HP-alpha-CyD < HP-beta- CyD. The bend structure due to a cis-double bond halfway along the acyl chain of oleic acid provided a better fit into the cavity of HP-beta-CyD, in which the double bond appears to be buried, and hence becomes less susceptible to oxidation. The rate and extent of nasal bioavailability of buserelin were remarkably increased by coadministration of oleic acid and HP-beta-CyD, compared with the sole use of the enhancer. This enhancement was ascribable to the lowering of both the enzymatic and physical barriers of the nasal epithelium to the peptide, probably through the facilitated transmucosal penetration of oleic acid solubilized in HP-beta-CyD.