1. The current I(Q) due to membrane charge movement and the threshold pulse duration t(th) required to produce microscopically just-detectable contraction were determined for pulses to a variety of membrane potentials in tendon-terminated short segments of cut frog skeletal muscle fibres voltage-clamped using a single gap technique.2. The time course Q(t) of membrane charge movement was calculated as the running integral of I(Q). The threshold charge Q(th) moved by pulses which produced just-detectable contraction was estimated as Q(t(th)).3. Q(th) was constant for pulses to potentials ranging from about -45 mV, the rheobase potential for contraction, to about -15 mV, where t(th) was about 9 msec. The mean Q(th) from fourteen fibres was 11.5 nC/muF, when the holding potential was about -100 mV.4. Prepulses of 50 msec which were themselves sub-rheobase for producing contraction decreased the t(th) for an immediately following test pulse. The total threshold charge moved during the prepulse and during t(th) of the test pulse was equal to Q(th) for the test pulse without prepulse.5. Items 3 and 4 above indicate that t(th) is determined by the time required to move a set amount of intramembrane charge, independent of the kinetics of the charge movement.6. Steady partial fibre depolarization to between -70 and -55 mV increased t(th) at all membrane potentials and elevated the rheobase potential for contraction. Slight further steady depolarization totally eliminated contraction.7. Steady partial depolarization decreased the total ON charge movement Q(ON) by about the same factor for pulses to all potentials tested.8. Q(th) for partially depolarized but still-contracting fibres remained approximately independent of membrane potential from rheobase to about 0 mV but was slightly less than Q(th) for the same fibres when fully polarized.9. Steady partial depolarizations which reduced the mean (+/-s.d.) ON charge movement Q(ON) to 60 +/- 8% of its value under full polarization reduced Q(th) to 86 +/- 11% of its full polarization value (n = 10). These steady partial depolarizations produced no change in the linear capacitance measured with hyperpolarizing pulses.10. Contraction was completely abolished by steady partial depolarizations which reduced Q(ON) to 41% of its value under full polarization (mean of three runs). The maximum value of Q(ON) was then 77% of the Q(th) value for the same fibres under full polarization.11. A prolonged tail, a shoulder, a second rising phase or an early relatively high flat segment were successively evident in the I(Q) records as the depolarizing pulse was successively increased to and beyond the rheobase potential for contraction. It was found that t(th) either coincided with or occurred slightly later than the start of such tails, shoulders or second rising phases.12. When test pulse I(Q) records with and without immediately preceding sub-rheobase prepulses were shifted in time so that their t(th) times coincided, the record with prepulse coincided with the later part of I(Q) without prepulse. This indicates that sub-rheobase prepulses moved the initial portion of the I(Q) that occurs during the test pulse alone, whereas they did not alter the latter portion of the test pulse I(Q).13. A model was developed which accounts for charge movement's voltage dependence and kinetics and for the relationship between charge movement and just-detectable contraction in both the fully polarized and partially depolarized states.14. The model proposes that Q be composed of two components. Component A is due to the voltage and time-dependent movement of charges between two sites located within the membrane and separated by a single energy barrier. Component B is instantaneously proportional to an integer power n of the fraction of component A charges which have crossed the barrier.15. The I(Q) time courses were best approximated using n = 3, with which both the relatively early and late portions of the experimental I(Q) time courses could be reproduced. The best theoretical records obtained with n = 3 still passed below the shoulders, second rising phases and later parts of the early constant phases in the various experimental I(Q) records. Theoretical records did fit accurately the I(Q) time courses observed under steady partial fibre depolarization. The relatively small current not reproduced by the model may be an electrical accompaniment of the activation of calcium release or the elevation of internal free calcium levels in the space between the transverse tubules (T-tubules) and the sarcoplasmic reticulum.