Experimental evidences indicating that the slow IPSP was produced from B cells in bullfrog sympathetic ganglia were presented. The slow IPSP was selectively depressed by the action of ouabain ; it, in comparison with the fast EPSP and slow EPSP in ganglionic B cells, was more sensitive to this drug. The muscarinic ACh-hyperpolarization, which corresponded to the slow IPSP, was also sensitive to ouabain. These results supported the hypothesis that the slow IPSP might be generated by an electrogenic sodium pump.