The effect of 4-aminopyridine (4-AP) on Kv channel activation has been extensively investigated, but its interaction with inactivation is less well understood. Voltage-clamp fluorimetry was used to directly monitor the action of 4-AP on conformational changes associated with slow inactivation of Shaker channels. Tetramethylrhodamine-5-maleimide was used to fluorescently label substituted cysteine residues in the S3-S4 linker (A359C) and pore (S424C). Activation- and inactivation-induced changes in fluorophore microenvironment produced fast and slow phases of fluorescence that were modified by 4-AP. In Shaker A359C, 4-AP block reduced the slow-phase contribution from 61 +/- 3 to 28 +/- 5%, suggesting that binding inhibits the conformational changes associated with slow inactivation and increased the fast phase that reports channel activation from 39 +/- 3 to 72 +/- 5%. In addition, 4-AP enhanced both fast and slow phases of fluorescence return upon repolarization (tau reduced from 87 +/- 15 to 40 +/- 1 ms and from 739 +/- 83 to 291 +/- 21 ms, respectively), suggesting that deactivation and recovery from inactivation were enhanced. In addition, the effect of 4-AP on the slow phase of fluorescence was dramatically reduced in channels with either reduced (T449V) or permanent P-type (W434F) inactivation. Interestingly, the slow phase of fluorescence return of W434F channels was enhanced by 4-AP, suggesting that 4-AP prevents the transition to C-type inactivation in these channels. These data directly demonstrate that 4-AP prevents slow inactivation of Kv channels and that 4-AP can bind to P-type-inactivated channels and selectively inhibit the onset of C-type inactivation.