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Experiments were performed to ascertain whether the monotonic (q beta) and delayed (q gamma) components of non-linear charge in skeletal muscle membranes form a sequential system, or are the result of separate, independent processes. The non-linear capacitance studied in a large number of fibres increased with fibre diameter. This dependence was attributable to tetracaine-sensitive (q gamma) but not to tetracaine-resistant (q beta and q alpha) charge. The kinetics and total quantity of q gamma charge moving in response to voltage steps from varying pre-pulse potentials to a fixed probe potential remained constant despite variations in the size of the early q beta decay. The kinetics of the delayed (q gamma) charging current obtained from a single 20 mV depolarizing step were compared with the sum of the responses to two 10 mV steps adding to the same voltage excursion. The respective transients superimposed only if one of the 10 mV steps did not reach the voltage at which q gamma first appears. In the two preceding experiments, total charge was conserved. These results are consistent with separate and functionally independent q beta and q gamma systems of potential-dependent charge, with q gamma residing in the transverse tubules and q beta on surface membrane. The findings can be discussed in terms of a contractile 'activator' with a steep sensitivity to voltage that begins only with depolarization beyond a level close to the actual mechanical threshold.