Plasma concentrations of atrial natriuretic factor (ANF) have been reported to be unchanged or increased in patients with essential hypertension. Head out of water immersion (HOI) in a thermoneutral bath induces diuresis and natriuresis, an increase in plasma ANF, and reductions in plasma renin activity and aldosterone concentrations. HOI was used in this study to stimulate the secretion of ANF, and compare its release in patients with essential hypertension (EH) (n = 14) and normotensive subjects (n = 13). Renal function changes induced by HOI were also monitored. HOI that lasted 2 h was compared with a control-seated period in each subject. Blood pressure was significantly reduced (P < .05) in normotensive controls from 112 +/- 3/74 +/- 2 to 100 +/- 3/61 +/- 2 mm Hg, and in patients with EH from 137 +/- 4/93 +/- 3 to 123 +/- 3/78 +/- 2 mm Hg. Plasma levels of ANF increased significantly (P < .05) in both groups from 5.9 +/- 1.3 to 16.3 +/- 3 pmol/L in normotensive controls and from 6.0 +/- 0.9 to 13.2 +/- 2.5 pmol/L in patients with EH. Plasma cyclic guanosine monophosphate concentrations increased more (P < .05) in the patients with EH (3.9 +/- 0.4 to 6.1 +/- 0.5 nmol/L) than in controls (3.4 +/- 0.3 to 4.8 +/- 0.4 nmol/L), whereas plasma renin activity levels decreased in controls (2.29 +/- 0.58 to 1.63 +/- 0.55 ng/mL/h) and to a greater degree in patients with EH (1.62 +/- 0.52 to 0.77 +/- 0.19 ng/mL/h, P < .05) by HOI.(ABSTRACT TRUNCATED AT 250 WORDS)