SUMMARY The interrelationship of blood pressure, cardiac output, and peripheral resistance was studied in Dahl "S" and "R" rats after 3 days on a high (8%) NaCl diet. Both "S" and "R" rats were nonnotensive when fed a normal (0.3%) NaCl diet. After 3 days of the high NaCl diet, the "R" rats remained normotensiTe (BP 112 mm Hg), while the "S" rats had an elevation of arterial pressure (BP 133 mm Hg) (p < 0.001). The cardiac outputs of both "S" and "R" rats were similar on the low NaCl diet. After 3 days high NaCl feeding, the cardiac output of the "R" rats rose 18% above "R" control level (p < 0.0001), while the peripheral resistance declined 14% below the "R" control level (p < 0.005), and the blood pressure (BP) did not change, a pattern quite contrary to the concept of "whole-body" autoregulation. With a similar 3-day high NaCl feeding in "S" rats, cardiac output (p < 0.005) and peripheral resistance (p < 0.05) both increased 10%, while BP rose 20%. After 7 days of high NaCl feeding, the cardiac output of the "S" rats had returned to normal, while blood pressure and peripheral resistance both continued to be elevated. This pattern of response in "S" rats could be compatible with the concept of "whole-body" autoregulation. However, since both NaCl hypertension and Goldblatt hypertension can occur in settings in which "whole-body" autoregulation appears not be to causally related, one cannot be certain whether "whole-body" autoregulation is playing a causal role in the mechanism of NaCl-induced hypertension in "S" rats. It is a striking dichotomy that 3 days of high salt feeding produces vasoconstriction in "S" rats and vasodilation "R" rats.