1. The electric potential fields around tuna ferri- and ferrocytochrome c were calculated assuming that (i) all of the lysines and arginines are protonated, (ii) all of the glutamic and aspartic acids and the terminal carboxylic acid are dissociated, and (iii) the haem has a net charge of +1e in the oxidized form. 2. Near the haem crevice high values for the potential (greater than +2.5 kT/e) are found. Consequently, electron transfer via the haem edge is favored if the oxidant or reductant is negatively charged. 3. The inhomogeneous distribution of charges leads to a dipole moment of 244 and 238 debye for oxidized and reduced tuna cytochrome c, respectively. Horse cytochrome c has dipole moments of 303 (oxidized) and 286 (reduced) debye. 4. A line through the positive and negative charge centres, the dipole axis, crosses the tuna cytochrome c surface at Ala 83 (positive part) and Lys 99 (negative part). The direction of the dipole axis of horse cytochrome c is very similar. Since the centre of the domain on the cytochrome c surface, which is involved in the binding to cytochrome c oxidase, is found at the beta-carbon of the Phe 82 in horse cytochrome c (Ferguson-Miller, S., Brautigan, D.L. and Margoliash, E. (1978) J. Biol. Chem. 253, 149--159) it is suggested that the direction of the dipole is of physiological importance. 5. The activity coefficients of horse ferri- and ferrocytochrome c were calculated as a function of ionic strength using a formula derived by Kirkwood (Kirkwood, J.G. (1934) J. Chem. Phys. 2, 351--361). 6. Due to the high net charge at pH 7.5 the influence of the dipole moments of horse ferri- and ferrocytochrome c on the respective activity coefficients can be neglected at I less than or equal to 50 mM. 7. Using the Brønsted relation the effect of ionic strength on reaction rates of horse cytochrome c was calculated. Good agreement is found between theory and experimental results reported in the literature.