A new vibrating probe-current voltage measuring system is described which enabled us to detect current-voltage curves in the acid and alkaline regions of Chara corallina (Klein ex Willd., em. R.D.W.). Extracellular current analysis, performed before and after the measurement of a current-voltage curve, established that the voltage-clamp protocol had no significant effect on the transport function of the plasma membrane, provided no action potential was triggered. This validated experimental system was then used to determine the reversal potential (- 450 mV) and the stoichiometry (1 H(+):ATP hydrolyzed) of the Chara H(+)-ATPase, which dominates the acid regions. Current-voltage curves of the acid regions almost saturated at values close to the resting potential, in the absence of exogenous buffer. Introduction of artificial buffers and-or HCO 3 (-) shifted the reversal potential of this area to more positive values. Furthermore, it was shown that the reversal potential (-120 mV) of the extracellular current in the alkaline band (passive H(+) channel) coincided with the threshold for the action potential. We propose that the action potential functions as a component of the spatial control system in the Chara cell.