The aldosterone and arterial pressure response to long-term infusion of two angiotensin II inhibitory analogues, [Sar1,Ala8]angiotensin II and [Sar1,Ile8]angiotensin II, and the angiotensin I-converting enzyme inhibitor, SQ 20,881, was studied in conscious dogs during sodium deficiency. Plasma aldosterone concentration (PAC), plasma cortisol concentration (PCC), and plasma renin activity (PRA) were determined by radioimmunoassay. In conscious dogs after dietary sodium restriction (5 mEq of Na+/day) for 21 days, PAC averaged 37.5 +/- 8.9 (mean +/- SE) ng/dl, PCC averaged 1.3 +/- 0.5 microng/dl, PRA averaged 3.23 +/- 0.42 ng/ml per hour, and arterial blood pressure (AP) averaged 103 +/- 5 mm Hg. During long-term infusion of [Sar1,Ala8]angiotensin II (5 microng/kg min-1), PAC averaged 34.7 +/- 8.5 ng/dl, PCC averaged 1.5 +/- 0.5 microng/dl, PRA averaged 16.4 +/- 3.1 ng/ml per hour, and AP averaged 88 +/- 5 mm Hg. During long-term infusion of [Sar1,Ile8]angiotensin II (5 microng/kg min-1), PAC averaged 45.8 +/- 12.6 ng/dl, PCC averaged 1.75 +/- 0.5 microng/dl, PRA averaged 13.6 +/- 4.3 ng/ml per hour, and AP averaged 86 +/- 5 mm Hg. During long-term infusion of angiotensin I-converting enzyme inhibitor (5 microng/kg min-1), PAC averaged 14.9 +/- 4.8 ng/dl, PCC averaged 1.75 +/- 0.5 microng/dl, PRA averaged 20.3 +/- 5.3 ng/ml per hour, and AP averaged 76 +/- 5 mm Hg. The intrinsic agonistic properties of the angiotensin II inhibitory analogues on the adrenal cortex negate their use for defining the role of the renin-angiotensin system in the regulation of aldosterone biosynthesis during sodium deficiency. The precipitous fall in aldosterone secretion and arterial blood pressure during long-term infusion of angiotensin I-converting enzyme inhibitor demonstrates the importance of the renin-angiotensin system in mediating aldosterone biosynthesis during sodium deficiency and the essential role of the renin-angiotensin-aldosterone system in the regulation of arterial pressure during sodium deficiency.