To study the hydraulic effects of subtotal immersion as a rehabilitative hydrotherapy, we examined the change in serum levels of atrial natriuretic peptide, catecholamine, cortisol and interleukins in 12 healthy volunteers. The subjects soaked in 42 degrees C water of 70 cm depth up to chin level in the upright seated position for 10 min. The serum level of atrial natriuretic peptide increased significantly 15 min after the start of subtotal immersion, though that of brain natriuretic peptide did not change. The serum dopamine level increased significantly 15 min after immersion, though neither the serum epinephrine nor norepinephrine levels did. In addition, 30 min after the start of immersion, the serum levels of atrial natriuretic peptide and dopamine decreased to those before immersion. The serum level of adrenocorticotropic hormone increased 15 min after immersion, though those of cortisol, interleukin-1beta and 6, and tumor necrotic factor-alpha did not change. It is suggested that 10-min head-out water immersion increased atrial natriuretic peptide partly due to increased venous return or right atrial load by hydraulic pressure.