Microelectrodes were used to measure membrane potential and intracellular potassium activity in surface epithelial cells (SEC) of frog (Rana esculenta) fundic gastric mucosa in vitro. Separate measurements were carried out by applying fine-tipped, single barrelled, KCl filled non-selective electrodes and liquid K+ -selective electrodes. Membrane potentials with respect to the mucosal and serosal surfaces, measured with non-selective electrodes, were -54.5 +/- 1.0 S.E mV (n = 59) and -73.0 +/- 1.1 S.E. mV (n = 59) respectively. The electrical potential difference referred to the mucosal surface, when measured with K+ -sensitive electrodes, was +21.2 +/- 0.8 S.E mV (n = 35), and intracellular K+ activity was 98.5 mmol/l. Assuming that intracellular and extracellular K+ activity coefficients are equal (gamma K = gamma K), the K+ concentration is 135.0 mmol/l. The K+ equilibrium potential, EK, was calculated as -90. mV i.e. more negative than both membrane potentials. This result indicates active potassium accumulation in the SEC and provides direct evidence of the presence of an active K+ pump in either both or in only one of the cell membranes.