Abstract2-Hydroxypropyl-β-cyclodextrin (HP-β-CyD) and 2,6-dimethyl-β-cyclodextrin (D-β-CyD) were studied for transdermal penetration enhancement of the cytochrome P450 inhibitor liarozole by an in-vivo transdermal absorption rat model. The mode of action of penetration enhancement was investigated by differential scanning calorimetry (DSC). In-vivo, HP-β-CyD, as a 20% aqueous solution, increased the absorption of liarozole approximately threefold and a 20% aqueous solution of D-β-CyD decreased the percutaneous absorption of liarozole in blood by a factor of 0·6. However, pretreatment with D-β-CyD (20%, 4 h) enhanced the transdermal absorption 9·4-fold. In the DSC experiments the thermal profile of human stratum corneum was practically unchanged after treatment with HP-β-CyD, but treatment with D-β-CyD revealed an interaction of D-β-CyD with the protein and lipid fraction. Thus the results from DSC and those from the permeability experiments revealed that D-β-CyD acts as a transdermal absorption enhancer by changing the stratum corneum barrier whereas HP-β-CyD influences the partitioning behaviour of the drug in the skin.