Previous studies demonstrated that ouabain-like compound (OLC) is widely distributed in mammalian species and is found in a variety of different tissues. Although much evidence suggests that OLC is endogenous to mammals, little information is available concerning physiological and/or pathophysiological roles for OLC. In this study, generic endogenous digitalis-like factor (E-DLF) was measured using an enzyme bioassay and the more specific OLC was determined using ouabain antisera in plasma drawn from dogs before and 30 and 120 min after massive volume expansion with isotonic saline. Plasma OLC was not changed by the saline load, whereas Na excretion was significantly elevated at the 30-min blood draw and remained elevated at the 120-min blood draw. Because renal exposure to OLC did not change with saline loading, it is unlikely that any portion of the profuse natriuresis in these animals could be attributed to OLC. In contrast, plasma E-DLF was higher after the saline load than before in each of four dogs at 30 and 120 min after the infusion. What portion of the profuse natriuresis can be attributed to E-DLF is unknown, although it is reasonable to assume that nanomolar levels of pump inhibitor contributed to the natriuresis to some degree.