Potassium channels play fundamental roles in physiology. Chemically diverse drugs bind in the pore region of K+ channels. Here, we homology-modeled voltage- and Ca2+-gated K+ channel BK and voltage-gated Kv1.3 using the X-ray structures of MthK and Kv1.2, respectively, and simulated the binding of d-tubocurarine in the inner pore of the channels. Monte Carlo minimization predicted that d-tubocurarine can bind in the open pore of both channels with its long axis parallel to the pore axis. The cationic groups of d-tubocurarine can displace K+ from the ion dehydration site at the selectivity filter. The predicted binding energy of d-tubocurarine in Kv1.3 is less preferable than in BK. To test this prediction, the currents through Kv1.3 and BK channels were measured in the absence and presence of d-tubocurarine. Results show that d-tubocurarine blocks current through Kv1.3 when applied from either side of the membrane only in millimolar concentrations (Kd= 1 mM), whereas half-blocking concentrations of the internally applied d-tubocurarine to BK are as low as approximately 8 microM. This indicates that the affinities of both external and internal d-tubocurarine to Kv1.3 are much lower than those to BK channels. Our study reveals the K+ dehydration site as a determinant of the d-tubocurarine receptor, predicts binding modes of d-tubocurarine in K+ channels, and suggests that the open pore in BK is wider than in Kv1.3. The results imply that MthK can be used for homology modeling of the pore region of channels activated by forces applied to the inner helices.