Phenylalkylamines like verapamil potently block the hKv1.3 channels. We suppose that there are similar structural properties in the inner vestibule of hKv1.3 and hKv1.2 channels since this part of the channels show high homology. Because the crystal structure of hKv1.2 has been described recently (Long et al., Science 309:897) we might use hKv1.2 channel as a model for further investigations in order to obtain more information about the structural properties of the hKv1.3 channel, especially in the inactivated state. For this purpose we used verapamil to characterise the phenylalkylamine binding site in the noninactivating hKv1.2 channel in comparison with the inactivated-state-reduced hKv1.3 H399T mutant. The whole cell configuration of the patch-clamp technique was used to examine the current properties as well as verapamil affinity. The IC₅₀ values for verapamil to block current were calculated by fitting a Hill function to the normalized steady state current data points after drug application. For hKv1.2 we found a K_d for steady state verapamil block of 2.7 mM and a K_d of 2.1 mM in the inactivated-state-reduced hKv1.3 H399T mutant. The similarity in the affinity of verapamil in both channels might indicate similar structural properties in the phenylalkylamine binding site indicating the validity of the use of hKv1.2 as a model for hKv1.3. Supported by grants from the 4SC AG (Martinsried) and the Wilhelm-Sander-Stiftung (2004.046.1).