Potassium (K / ) conductances are known to be involved in cell proliferation of a number of nonexcitable cell types. The nature of the mechanism by which K / channel inhibition reduces cell proliferation has remained elusive despite intensive search. We investigated whether such a phenomenon could be demonstrated in excitable cells, using the GH3 pituitary cell line as a cell model. Our aims were: 1) to study the effect of K / channel inhibition on the proliferation of GH3 cells; and 2) to investigate the putative intracellular signals involved in this inhibition. Tetraethylammonium chloride (TEA), a blocker of the calcium (Ca 2/ )-dependent K / conductances of GH3, was found to reversibly inhibit cell proliferation, as measured by 3 H-thymidine incorporation. Cell cycle block specifically occurred at the G1/S phase of the cell cycle. This inhibition of proliferation was observed for 1-4 mM TEA, which suppressed most of the Ca 2/ -activated K / current and part of the inward rectifying K / current, as shown by electrophysiological experiments. Increasing extracellular K / concentrations with KCl also inhibited cell proliferation in a dose-dependent manner. Both TEA and KCl depolarized the cells and increased intracellular Ca 2/ levels ([Ca 2/ ]i), showing that, in this type of excitable cell, inhibition of cell proliferation can be associated with elevated Ca 2/ levels. Ca 2/ and membrane resting potential (MRP) were considered as possible messengers of this inhibition. Our results suggest that cell cycle arrest of GH3 cells by K / channel block probably involves an additional pathway, distinct from those of Ca 2/ and MRP.