Changes in the conduction velocity and subsequent conduction block were characterized following impulse activity in single C fibers of rat sciatic nerves. C fibers which had the same resting conduction velocities often exhibited quite different profiles of the activity-dependent latency change and/or conduction block following impulses. The results imply underlying variation among C fibers in the activity-dependent excitability changes, especially in the build-up and recovery of the hypoexcitable phases.