The rates of hydrolysis of prostacyclin (PGI2) and its methyl ester (PGI2Me) in aqueous solution were significantly retarded by α-, β-, and γ-cyclodextrins (α-, β-, and γ-CyDs), and showed characteristic saturation kinetics and competitive inhibition. The deceleration effects of CyDs on the hydrolysis of PGI2Me were about 3 times larger than those on the hydrolysis of PGI2. The importance of the spatial relationship between the host and guest molecules was reflected in the kinetically determined stability constant (Kc) for these inclusion complexations. To elucidate the deceleration mechanism of the CyDs, the effects of pH, solvent and temperature on the hydrolysis rate were studied. The protolytic dissociation of the terminal carboxylic acid moiety of PGI2 was suppressed by the binding to CyDs, depending upon the magnitude of the Kc value. Thermodynamic activation parameters suggested that the deceleration mechanism of CyDs in the case of PGI2 was somewhat different from that for PGI2Me, which may be due to different modes of inclusion.