Amounts and temporal changes of the release of the tracer ions K+ (86Rb+),22Na+, and36Cl− as well as of H+ in the course of action potentials inAcetabularia have been recorded. New results and model calculations confirm in quantitative terms the involvement of three major ion transport systemsX in the plasmalemma: Cl− pumps, K+ channels, and Cl− channels (which are marked in the following by the prefixes,P, K andC) with their equilibrium voltagesXVe and voltage/time-dependent conductances, which can be described by the following, first approximation. Let the maximum (ohmic) conductance of each of the three populations of transporter species be about the same (PL,KL,CL=1) but voltage gating be different: the pump (pVe about −200 mV) being inactivated (open,o→closed,c) at positive going transmembrane voltages,Vm; the K+ channels (KVe about −100 mV) are inactivated at negative goingVm; and the Cl− channels (CVe: around 0 mV), which are normally closed (c) at a restingVm (nearPVe) go through an intermediate open (o) state at more positiveVm before they enter a third “shut” state (s) in series. Model calculations, in which voltage sensitivities are expressed by the factorf=exp(VmF/(2RT)), simulate, the action potential fairly well with the following parameters (PKco∶10/f ks−1,PKoc∶1000·f ks−1,KKco∶200·f ks−1,Kkoc∶2/f ks−1,cKco∶500·f ks−1,CKoc∶5/f ks−1,CKso∶0.1/f ks−1,Ckos∶20·f ks−1). It is also shown that the charge balance for the huge transient Cl− efflux, which frequently occurs during an action potential, can be accounted for by the observation of a corresponding release of Na+.