The electron exchange kinetics of horse heart cytochrome c (Cyt c) at 4,4'-bipyridyl- and 4,4'-bipyridyl-disulfide-modified Au electrodes has been studied for the first time under variable pressure conditions (up to 150 MPa), by using fast scanning cyclic voltammetry. A positive activation volume of +6.1 +/- 0.5 cm(3) mol(-1) was determined from the pressure dependence of the heterogeneous standard rate constant in both cases. This value is similar to that for the homogeneous Cyt c self-exchange process, predicted from the cross-reaction treatment. A careful analysis based on an extended version of the contemporary charge-transfer theory indicates that the process most probably takes place through an adiabatic ("protein friction") charge-transfer mechanism in which the positive volume of activation results from the pressure-induced increase of the protein's intrinsic viscosity (decrease of the characteristic relaxation mobility), which is also in remarkable agreement with earlier results from studies in which the viscosity was varied directly. This approach allows for variation of the internal protein viscosity without significant alteration of the properties (viscosity, diffusion coefficients) of the aqueous medium.