Based on the structure of the KcsA potassium channel, the Shaker K+ channel is thought to have, near the middle of the membrane, a cavity that can be occupied by a permeant or a blocking cation. We have studied the interaction between cations in the cavity and the activation gate of the channel, using a set of monovalent cations together with Shaker mutants that modify the structure of the cavity. Our results show that reducing the size of the side chain at position 470 makes it possible for the mutant channel, unlike native Shaker, to close with tetraethylammonium (TEA+) or the long-chain TEA-derivative C10+ trapped inside the channel. Neither I470 mutants nor Shaker can close when N-methyl-glucamine (NMG+) is in the channel, even though this ion is smaller than C10+. Apparently, the carbohydrate side chain of NMG+ prevents gate closing. Gating currents recorded from Shaker and I470C were measured in the presence of different intracellular cations to further analyze the interaction of cations with the gate. Our results suggest that the cavity in Shaker is so small that even permeant cations like Rb+ or Cs+ must leave the cavity before the channel gate can close.