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1. Models are presented for (a) HK ATPase acting in the presence of K and Cl conductances; (b) a pH regulatory system where Na/H exchange is regulated directly by second messenger and the anion exchanger is activated secondarily to the rise in cell pH; (c) vesicle fusion and K and Cl conductances activation in the gastric parietal cell. 2. It is suggested that H transport involves protonation and deprotonation of histidine groups as well as the motion of these groups relative to the membrane barrier. 3. The HK ATPase would have a voltage generating and voltage sensitive step in the forward direction. 4. Given net electroneutrality the K transport reaction would also be charge translocating and voltage sensitive.