A major biologically active endogenous digitalis-like factor in the mammalian body may be an isomer of ouabain (ouabainlike compound, OLC). However, the exact role of OLC in sodium homeostasis is still unclear, and acute isotonic volume expansion does not enhance the secretion of OLC. We tested the hypothesis that OLC may be more important in the response to acute hypertonic NaCl load rather than isotonic volume expansion. We injected intraperitoneally 2 mL of 20% NaCl solution into male Wistar rats (n = 34) and measured OLC levels in plasma, hypothalamus, pituitary, and adrenal at baseline (n = 10) and 1, 2, and 4 hours (n = 8 for each). In response to hypertonic NaCl loading, plasma Na-K ratio was elevated at 2 and 4 hours (P < .01). OLC levels in pituitary increased (P < .01) at 1 hour. Thereafter, plasma OLC levels increased at 2 and 4 hours (P < .05; basal, 75 +/- 11 pmol/L [+/- SEM]; 1 hour, 55 +/- 11; 2 hours, 130 +/- 24; 4 hours, 156 +/- 20). Concomitantly, OLC levels in adrenal increased at 2 and 4 hours (P < .01; basal, 1.7 +/- 0.2 pmol/g; 1 hour, 4.5 +/- 0.9; 2 hours, 5.0 +/- 0.7; 4 hours, 6.8 +/- 2.2). A significant correlation was observed between OLC levels in plasma and adrenal (P <.05). Plasma Na-K ratio positively correlated with OLC levels in plasma (r = .51, P < .01) and adrenal (r = .48, P < .01). Similar injection of physiological saline solution or hypertonic sucrose solution in physiological saline did not increase OLC levels in plasma and tissues. These findings indicate the elevation of OLC levels in plasma, pituitary, and adrenal in response to acute hypertonic NaCl load in rats and suggest that OLC may be involved in the response to the hypernatremic state. (Hypertension. 1997;30[part 1]:94-98.)