After preliminary conditioning depolarizations, single muscle fibers of the frog were tested for ability to contract in response to depolarization by 100 mM K+ Ringer solution. Denervated fibers (6-42 days) lose their ability to produce a 100 mM K+ contracture more rapidly than do control fibers. This decrease in 100 mM K+ contracture size (inactivation) is dependent on length of exposure to and magnitude of the conditioning depolarization and on the calcium concentration in the external medium. At 0.4 mM Ca++, the inactivation is 3 times faster than at 1.5 mM Ca++. The rate of contracture loss is not correlated with fiber diameter or the number of days after failure of neuromuscular transmission, and the preliminary conditioning depolarizations do not affect the rate of terminal relaxation from the 100 mM K+ contractures.