Inclusion complex formations of spironolactone (SP) with three cyclodextrins (α-, β-, γ-CyDs) in aqueous solution and in the solid state were studied by the solubility method, by spectroscopic methods (UV, CD, IR) and by X-ray diffractometry, and their modes of interaction were assessed. The solid complexes of SP with β- and γ-CyDs were obtained in molar ratios of 1 : 2 and 2 : 3, respectively, and their dissolution, membrane permeation and oral absorption properties were examined. The rates of dissolution and permeation through a cellophane membrane in water were significantly increased by inclusion complexation (γ-CyD complex>β-CyD complex>SP alone), depending upon the solubility of the test samples. The serum levels of SP following oral administration of CyD complexes were found to be greater than those after administration of SP alone. The results indicated that the γ-CyD complex rather than β-CyD complex may have great utility as a faster dissolving form of SP able to produce higher serum levels.