The jejunal tract of rat intestine, everted and incubated in vitro at 28 degrees C for 2 hr in Krebs-Ringer bicarbonate solution, was used to test the existence of a ouabain-insensitive sodium pump. Cell water, Na, and K together with Na, fluid, K, and lactate transported into the serosal compartment were determined and, under control conditions, the tested parameters were found constant in time. By blocking the Na-K pump with 20 mM ouabain in the serosal compartment, the enterocyte lost K and gained Na, but the cell volume did not vary. Moreover, the transport of Na, fluid, and lactate, although lower, was constant for 2 hr. When ethacrynate was added or when the ATP supply was blocked by adding 2,4-dinitrophenol plus iodoacetate, the cell swelled and the transport of Na and fluid stopped. These results are interpreted as suggesting the existence of a ouabain-insensitive Na pump, in addition to the well-known Na-K pump.