Hyperpolarizing conditioning pulses delay the onset of potassium channel current in voltage-clamped myelinated nerve fibers. Both the development of and recovery from this conditioning are approximately exponential functions of time: the time constants are functions of the conditioning voltage. The delay is larger and develops faster for more hyperpolarized conditioning pulses. The magnitude of the delay (but not the rate of development or recovery) depends upon the test potential-small test depolarizations produce larger delays than large depolarizations. The currents with and without the conditioning pulse cannot be made to superimpose by a simple time translation.