We have developed a simple laboratory exercise that uses an inexpensive dialysis membrane (molecular weight cutoff = 100) to illustrate the generation of membrane potentials (V_m) across plasma membranes of animal cells. A piece of membrane ~2.0 cm² is mounted in an Ussing-like chamber. One chamber half is designated cytosol and the other half external. Chamber sidedness helps students relate their findings to those of real cells. As in real cells, outward directed K⁺ concentration gradients [high cytosolic K⁺ concentration ([K⁺]_c) and low extracellular K⁺ concentration] generate cytosol electrically negative V_m with a slope of approximately -45 mV/decade change in [K⁺]_c. The polarity of V_m reflects the outward flow of potassium ions because flow of the larger counterion, H₂PO₄^-, is restricted by the pores in the membrane. A slope less than Nernstian (<59 mV/decade) suggests that the membrane is slightly permeable to H₂PO₄^-. Importantly, this facilitates teaching the use of the Nernst equation to quantify the relationship between ion concentration ratios across membranes and magnitude of V_m. For example, students use their data and calculate a permeability ratio P_K/P_H2PO4 that corresponds to a slope ~24% less than Nernstian. This calculation shows that Nernstian slopes are achieved only when permeability to the counterion is zero. Finally, students use the concept of membrane capacitance to calculate the number of ions that cross the membrane. They learn where these ions are located and why the bulk solutions conform to the principle of electroneutrality.AM. J. PHYSIOL. 277 (ADV. PHYSIOL. EDUC. 22): S51-S59, 1999.