Oscillations in the activity of an electrogenic Na pump has been suggested as the ionic mechanism underlying the intestinal control potential (slow wave). We investigated the electrogenicity of this pump in rabbit jejunal smooth muscle. Potassium admission to Na-rich tissues caused a large increase in membrane potential which after 10--20 min decreased toward values comparable with those of normal tissues. This hyperpolarization far exceeded EK and could be prevented by cooling or by ouabain. No hyperpolarization occurred upon K admission to Li-rich tissues in the absence of Na. Thus, the pump in this tissue can operate electrogenically. Goldman's equation was modified so as to account for the pump's contribution to the membrane potential. Using this equation, the calculated contribution of the pump, under normal "steady-state" conditions, is unlikely to exceed a few millivolts. It is concluded that although the pump in this tissue can be electrogenic, its contribution may be smaller than that required if the intestinal control potential resulted from rhythmic turning off and on of the electrogenic Na pump.