The efflux of radioactive choline from exclusively neuronal or glial cell cultures was dependent upon the concentrations of choline present in the cells and in the incubation medium, suggesting the possible presence of a homoexchange phenomenon between influx and efflux. The ionic dependence of the outward movement of choline from these cells showed that is could be stimulated by high K+ concentrations and by the absence of Ca2+. In glial cells, however, the efflux of choline was increased with a much lower concentration of K+ compared to neurons. The result may suggest that during nerve stimulation the release of K+ from neurons could stimulate, from glia, the efflux of choline which would then be taken up in neurons.