Short muscle fibers (less than 1.5 mm) from the m. lumbricalis IV digiti of Rana pipiens were voltage-clamped at -100 mV with a two-microelectrode technique, in normal Ringer's solution containing 10(-6) g/ml tetrodotoxin. The activation curve relating peak tension to membrane potential could be shifted toward more negative or less negative potential values by hyperpolarizing or depolarizing the fiber membrane to -130, -120, or -70 mV, respectively, which indicates that contractile threshold depends on the fiber membrane potential. Long (greater than 5 s) depolarizing (90 mV) pulses induce prolonged contractile responses showing a plateau and a rapid relaxation phase similar to K contractures. Conditioning hyperpolarizations prolong the time course of these responses, while conditioning depolarizations shorten it. The shortening of the response time course, which results in a decrease of the area under the response, is dependent on the amplitude and duration of the conditioning depolarization. Depending on the magnitude and duration, a conditioning depolarization may also reduce peak tension. When the area under the response is reduced by 50%, the level of membrane potential also affects the repriming rate. During repriming, peak tension is restored before the contracture area. Thus, when peak tension is reprimed to 80%, the area is reprimed by 50% of its normal value. Repriming has a marked temperature dependency with a Q10 higher than 4. These results are compatible with the idea that an inactivation process, voltage and time dependent, regulates the release of calcium from the sarcoplasmic reticulum during these responses.