The diffusion potential in taenia coli cells calculated from the intra- and extracellular ion concentrations and from the calculated membrane permeabilities is less negative than the measured membrane potential. This discrepancy could be due to a continuous contribution of an electrogenic ion pump to the membrane potential. This hypothesis is supported by the finding that the sum of the active ion fluxes is sufficiently large to generate a potential of -20 mV across the membrane. Moreover, an unequivocal electrogenic component of the membrane potential has been demonstrated during maximal activation of the ion pump in K-depleted cells. The importance of this electrogenic component depends on the membrane resistance, a parameter which is very much affected by the experimental procedure. The active Na-K exchange is stimulated by [Na]<latex>$_i$</latex> and [K]<latex>$_o$</latex> and is inhibited by [Na]<latex>$_o$</latex>. Furthermore, it has been observed that 47% of the Na-exchange of Na-enriched tissues is due to Na exchange diffusion.