We have studied the relation between permeation and recovery from N-type or ball-and-chain inactivation of ShakerB K channels. The channels were expressed in the insect cell line Sf9, by infection with a recombinant baculovirus, and studied under whole cell patch clamp. Recovery from inactivation occurs in two phases. The faster of the two lasts for approximately 200 ms and is followed by a slow phase that may require seconds for completion. The fast phase is enhanced by both permeant ions (K+, Rb+) and by the blocking ion Cs+, whereas the impermeant ions (Na+, Tris+, choline+) are ineffective. The relative potencies are K+ > Rb+ > Cs+ > NH4+ >> Na+ approximately choline+ approximately Tris+. Ion permeation through the channels is not essential for recovery. The results suggest that cations influence the fast phase of recovery by binding in a site with an electrical distance greater than 0.5. Recovery from fast inactivation is voltage-dependent. With Na+, choline+, or Tris+ outside, about 15% of the channels recover in the fast phase (-80 mV), and the other 85% apparently enter a second inactivated state from which recovery is very slow. Recovery in this phase is not influenced by external ions, but is speeded by hyperpolarization.