FOLLOWING recent publications in Nature on hydrogen ion concentrations and flows in mitochondria1-5, Baum6 (see preceding communication) has aptly drawn attention to the fact that the chemiosmotic hypothesis does not require a significant pH. difference across the mitochondrial membrane system7. In the original outline of the hypothesis8 it was shown that the major component of the protonmotive force (p.m.f.) should be a membrane potential. Admitting therefore that the experimental facts may be consistent with the driving of adenosine triphosphate (ATP) synthesis by a proton current that flows through the reversible ATPase system in the cristae membrane under a mainly electrical p.m.f., Baum6 has asked how the very small number of free protons in the inner aqueous phase of a mitochondrion or sub-mitochondrial particle can permit the flow of an effective proton current, and whether there may not be special energetic obstacles to proton translocation in such small systems.